go-tfe is pinned to branch run-tasks-integration pending API changes until run tasks support in the CLI is closer to release

pull latest changes from go-tfe branch and use use new field name that previously was incorrectly named TaskStage

This change will await the completion of pre-apply run tasks if they
exist on a run and then report the results.

It also adds an abstraction when interacting with cloud integrations such
as policy checking and cost estimation that simplify and unify output,
although I did not go so far as to refactor those callers to use it yet.

There was an unintended regression in go-getter v1.5.9's GitGetter which
caused us to temporarily fork that particular getter into Terraform to
expedite a fix. However, upstream v1.5.10 now includes a
functionally-equivalent fix and so we can heal that fork by upgrading.

We'd also neglected to update the Module Sources docs when upgrading to
go-getter v1.5.9 originally and so we were missing documentation about the
new "depth" argument to enable shadow cloning, which I've added
retroactively here along with documenting its restriction of only
supporting named refs.

This new go-getter release also introduces a new credentials-passing
method for the Google Cloud Storage getter, and so we must incorporate
that into the Terraform-level documentation about module sources.

Use the latest k8s.io packages from v0.21

- Fix tests and remove commented code
- Remove parallel for some flaky tests
- Add README

cty: The documented definition and comparison logic of cty.Number is now
refined to acknowledge that its true range is limited only to values
that have both a binary floating point and decimal representation,
because cty values are primarily designed to traverse JSON serialization
where numbers are always defined as decimal strings.

In particular, that means that two cty.Number values now always compare
as equal if their representation in JSON (under cty's own JSON encoder)
would be equal, even though the decimal approximation we use for that
conversion is slightly lossy. This pragmatic compromise avoids confusing
situations where a round-trip through JSON serialization (or other
serializations that use the same number format) may produce a value that
doesn't compare equal to the original.

This new definition of equals should not cause any significant behavior
change for any integer in our in-memory storage range, but may cause
some fractional values to compare equal where they didn't befo...

* Add test for tfc to tfc mgiration
* Fix old tests, and remove unused code.

Run variables allow the run API to accept input variables not found
in the configuration slug (the file-based ones plus workspace vars)

* determining source or destination to cloud
* handling single to single state migrations to cloud,
using a name strategy or a tags strategy
* Add end-to-end tests for state migration.

These changes include additions to fulfill the interface for the client
mock, plus moving all that logic (which needn't be duplicated across
both the remote and cloud packages) over to the cloud package under a
dedicated mock client file.

Stop using panicwrap, and execute terraform in the main process.

We don't use this library anywhere else in Terraform, and this backend was
using it only for trivial helpers that are easy to express inline anyway.
The new direct code is also type-checkable, whereas these helper functions
seem to be written using reflection.

This gives us one fewer dependency to worry about and makes the test code
for this backend follow a similar assertions style as the rest of this
codebase.

Bumps [github.com/golang/protobuf](https://github.com/golang/protobuf) from 1.5.0 to 1.5.2.
- [Release notes](https://github.com/golang/protobuf/releases)
- [Commits](https://github.com/golang/protobuf/compare/v1.5.0...v1.5.2

)

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updated-dependencies:
- dependency-name: github.com/golang/protobuf
  dependency-type: direct:production
  update-type: version-update:semver-patch
...
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This also transitively upgrades golang.org/x/sys and golang.org/x/net,
but there do not seem to be any significant changes compared to the
commits we were previously using.

Bumps [github.com/xanzy/ssh-agent](https://github.com/xanzy/ssh-agent) from 0.2.1 to 0.3.1.
- [Release notes](https://github.com/xanzy/ssh-agent/releases)
- [Commits](https://github.com/xanzy/ssh-agent/compare/v0.2.1...v0.3.1

)

---
updated-dependencies:
- dependency-name: github.com/xanzy/ssh-agent
  dependency-type: direct:production
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Bumps [github.com/agext/levenshtein](https://github.com/agext/levenshtein) from 1.2.2 to 1.2.3.
- [Release notes](https://github.com/agext/levenshtein/releases)
- [Commits](https://github.com/agext/levenshtein/compare/v1.2.2...v1.2.3

)

---
updated-dependencies:
- dependency-name: github.com/agext/levenshtein
  dependency-type: direct:production
  update-type: version-update:semver-patch
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CVE-2020-26160 is a high-severity advisory reported against this module.

The dgrijalva package is no longer maintained but our legacy etcv2 backend
depends on it indirectly, via go.etcd.io/etcd/client. The golang-jwt
package is the blessed successor of the original, and has a v3 line which
is compatible with the v3 line of dgrijalva, and so through this replace
we can get a fix for the advisory without other significant behavior
change.

We've preserved the etcdv2 backend as-is on a best-effort basis in order
to support anyone who is already using it, but recommend that users switch
to etcdv3 or to some other backend for ongoing use.

We also have future plans to make state storage be a matter for provider
plugins rather than built in to Terraform CLI, at which point this backend
will either become obsolete or be factored out into its own plugin, at
which point we can remove this "replace" directive and the associated
dependency altogether.

Bumps [honnef.co/go/tools](https://github.com/dominikh/go-tools) from 0.0.1-2020.1.4 to 0.3.0-0.dev.
- [Release notes](https://github.com/dominikh/go-tools/releases)
- [Commits](https://github.com/dominikh/go-tools/compare/v0.0.1-2020.1.4...v0.3.0-0.dev

)

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This is just a routine upgrade to the latest v1 release.

Bumps [github.com/mitchellh/copystructure](https://github.com/mitchellh/copystructure) from 1.0.0 to 1.2.0.
- [Release notes](https://github.com/mitchellh/copystructure/releases)
- [Commits](https://github.com/mitchellh/copystructure/compare/v1.0.0...v1.2.0

)

---
updated-dependencies:
- dependency-name: github.com/mitchellh/copystructure
  dependency-type: direct:production
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