We previously had this functionality available for cached packages in the
providercache package. This moves the main implementation of this over
to the getproviders package and then implements it also for PackageMeta,
allowing us to compute hashes in a consistent way across both of our
representations of a provider package.
The new methods on PackageMeta will only be effective for packages in the
local filesystem because we need direct access to the contents in order
to produce the hash. Hopefully in future the registry protocol will be
able to also provide hashes using this content-based (rather than
archive-based) algorithm and then we'll be able to make this work for
PackageMeta referring to a package obtained from a registry too, but
hashes for local packages only are still useful for some cases right now,
such as generating mirror directories in the "terraform providers mirror"
command.
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