This reverts commit 3cb12b5a.

Fixes #27506

Add a new flag `-lockfile=readonly` to `terraform init`.
It would be useful to allow us to suppress dependency lockfile changes
explicitly.

The type of the `-lockfile` flag is string rather than bool, leaving
room for future extensions to other behavior variants.

The readonly mode suppresses lockfile changes, but should verify
checksums against the information already recorded. It should conflict
with the `-upgrade` flag.

Note: In the original use-case described in #27506, I would like to
suppress adding zh hashes, but a test code here suppresses adding h1
hashes because it's easy for testing.
Co-authored-by: Alisdair McDiarmid <alisdair@users.noreply.github.com>

During the language/CLI docs reorg, we noticed several pages that were no longer
viable; some were redundant, some useless, and some just very obsolete.

Since we were trying to avoid breaking links at the time, we opted to remove
them from the navs and leave them as "ghost pages" — still accessible, but not
findable.

This commit finally cleans these ghosts up and updates any remaining links to
relevant modern pages. Bustin' makes me feel good. 👻🚫

These links largely still go somewhere useful, but they have some kind of issue
revealed by our new link checker:

- Some of them point to a stale URL that redirects, and can be updated to the
  new destination.
- Some of them point to anchors that don't exist (anymore?) in the destination.
- Some of them end up redirecting unnecessarily due to how the server handles
  directory URLs without trailing slashes. Sorry, I know that's pointless, just,
  humor me for the time being so we can get our CI green. 😭

In a couple cases, I've added invisible anchors to destination pages, either to
preserve an old habit or because the current anchors kind of suck due to being
particularly long or meandering.

As of Terraform 0.13+, the get-plugins command has been
superceded by new provider installation mechanisms, and
general philosophy (providers are always installed, but
the sources may be customized). Updat the init command
to give users a warning if they are setting this flag,
to encourage them to remove it from their workflow, and
update relevant docs and docstrings as well

This includes both the main documentation about the lock file itself and
changes to related documentation about Terraform commands that interact
with the lock file.

We will likely continue to update this first pass of documentation as we
get feedback and questions during the prerelease period.

* website: Update all Learn crosslinks

The URL structure on Learn recently changed, so it's time to update some URLs.
Co-authored-by: Tu Nguyen <im2nguyen@users.noreply.github.com>

This new option is intended to address the previous inconsistencies where
some older subcommands supported partially changing the target directory
(where Terraform would use the new directory inconsistently) where newer
commands did not support that override at all.

Instead, now Terraform will accept a -chdir command at the start of the
command line (before the subcommand) and will interpret it as a request
to direct all actions that would normally be taken in the current working
directory into the target directory instead. This is similar to options
offered by some other similar tools, such as the -C option in "make".

The new option is only accepted at the start of the command line (before
the subcommand) as a way to reflect that it is a global command (not
specific to a particular subcommand) and that it takes effect _before_
executing the subcommand. This also means it'll be forced to appear before
any other command-specific arguments that take file paths, which hopeful...

Co-authored-by: Tu Nguyen <im2nguyen@users.noreply.github.com>

This document now lives at:

- https://learn.hashicorp.com/terraform/development/running-terraform-in-automation

...and terraform.io has been redirecting to there for quite a while. This commit
removes the extra copy so that the text of the two versions doesn't drift, and
updates existing links to point to the new location.

...and one other reference to the application data directory.

Context:

https://docs.microsoft.com/en-us/windows/desktop/shell/knownfolderid#folderid_roamingappdata

In newer Windows versions, the folder accessible as `%APPDATA%` (and via various
APIs) is actually at something like "documents and settings\user\application
data\roaming", while earlier versions omit the "\roaming" part of the path. This
means you can confuse people by referring to the "application data" directory by
its human name, because "roaming" is the real application data directory, but it
looks like a subdirectory of "application data".

Thus, it's less confusing to just use the `%APPDATA%` variable, with the added
benefit that you can copy and paste the path and it'll just work in most places.

website: Reconcile docs about plugin discovery and downloading

I'm attempting to keep things simple for normal users while making sure we've
got the full behavior written down somewhere for plugin developers.

This commit doesn't stand alone; it's paired with a commit in the
terraform-website repo, to deal with some related content in the "extend"
section.

Sentence correction

Fix typo and replace some "it's" with "it is", according to rules for written English.

Reword the paragraph to match the voice and point of view of the rest of
the section. Use the term general termn "plugin" rather than "provider"

The docs did not mention that it is possible to provide overrides for specific
plugins by placing them into a `terraform.d/plugins/os_arch/` directory inside
the working dir.

Closes #15727.

This restores the earlier behavior of the first positional argument to
terraform init in 0.9, but as a command line option.

The positional argument was removed to improve consistency with other
commands that take a working directory as their first positional argument.
It was originally intended that this functionality would return in a
later release along with some other general improvements to Terraform's
module handling, but we're introducing here an interim solution that
uses the existing module source concept, to allow for easier porting of
workflows that previously depended on the automatic copy behavior.

In a future release this feature may change again as the module
improvements design firms up, but we expect it to be broadly compatible
with this temporary state.

The "terraform init" command has a lot of different functionality now,
making it hard to follow all of the options in the previous presentation.
Instead, here we describe each of the steps and its associated options
separately, hopefully making it easier to understand what each option
relates to.

In addition, much of the detail around backend partial configuration is
factored out into the backend configuration page, where it seems more
"at home"; previously it felt hard to follow exactly how partial
configuration would be used, due to the information on it being split over
two different pages.

This repo now contains only the core docs, with other content moving elsewhere.

"environment" is a very overloaded term, so here we prefer to use the
term "working directory" to talk about a local directory where operations
are executed on a given Terraform configuration.

add -reconfigure, and fix ordering of the other flags