This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
We previously had some tests for some happy paths and a few specific
failures into an empty directory with no existing locks, but we didn't
have tests for the installer respecting existing lock file entries.
This is a start on a more exhaustive set of tests for the installer,
aiming to visit as many of the possible codepaths as we can reasonably
test using this mocking strategy. (Some other codepaths require different
underlying source implementations, etc, so we'll have to visit those in
other tests separately.)
Martin Atkins