We previously skipped this one because it wasn't strictly necessary for
replicating the old "terraform init" behavior, but we do need it to work
so that things like the -plugin-dir option can behave correctly.
Linking packages from other cache directories and installing from unpacked
directories are fundamentally the same operation because a cache directory
is really just a collection of unpacked packages, so here we refactor
the LinkFromOtherCache functionality to actually be in
installFromLocalDir, and LinkFromOtherCache becomes a wrapper for
the installFromLocalDir function that just calculates the source and
target directories automatically and invalidates the metaCache.
When a provider source produces an HTTP URL location we'll expect it to
resolve to a zip file, which we'll first download to a temporary
directory and then treat it like a local archive.
When a provider source produces a local archive path we'll expect it to
be a zip file and extract it into the target directory.
This does not yet include an implementation of installing from an
already-unpacked local directory. That will follow in a subsequent commit,
likely following a similar principle as in Dir.LinkFromOtherCache.
Martin Atkins