This "kitchen sink" commit is mainly focused on supporting "targets" as
a new sub-category of addresses, for use-case like the -target CLI option,
but also includes some other functionality to get closer to replacing
terraform.ResourceAddress and fill out some missing parts for representing
various other address types that are currently represented as strings
in the "terraform" package.

This gives us a convenient way to convert back to a string representation
that matches what the user would've written in configuration.

This package is intended to contain all the functionality for parsing,
representing, and formatting addresses of objects within Terraform.

It will eventually subsume the responsibilities of both the
InterpolatedVariable and ResourceAddress types in the "terraform" package,
but for the moment is just a set of types for representing these things,
lacking any way to parse or format them. The remaining functionality
will follow in subsequent commits.