We've been gradually chipping away at how much we use go-getter for source
packages, because it's generally been a bit of a nightmare and sharing it
with other codebases means that any time someone wants to change something
we end up needing to find some way to prevent it breaking Terraform's
compatibility promises.
Here we make one further step: Terraform owns the "detectors" idea that
deals with source address normalization, and now always produces
fully-qualified addresses for go-getter to chew on only for the getting
and decompressing steps.
Retaining go-getter for the actual getting part is helpful because we can
then benefit from security fixes upstream, but Terraform owning the first
layer of parsing means that we can fix in place the definition of what
"module source address" syntax means, and thus we can avoid having
everything in this codebase indirectly depend on go-getter just because it
wants to parse module source addresses.
Now only the module installer actually depends indirectly on go-getter,
which finally disconnects go-getter's subtree from all of the remote state
backend dependency graphs.
Our package addrs ends up getting imported from just about ever other
package in Terraform, because it contains the types we use to talk about
various different kinds of objects. Therefore we typically try to keep its
transitive dependency graph relatively small, because anything it depends
on becomes an indirect dependency of nearly everything else.
A while back we moved the module source address models into package addrs,
which also brought with them the code for parsing strings to produce those
addresses. Unfortunately, remote module source addresses are defined using
the external dependency go-getter, which is pretty heavy itself and also
brings with it numerous other external dependencies such as the AWS SDK,
the Google Cloud Platform SDK, etc.
Since only relatively few packages actually need to _parse_ source
addresses -- with most either not caring about them at all or only
consuming addresses that were already parsed by someone else -- we'll
move the parser functions into their own package, while keeping the
resulting address types in package addrs.
This does still retain the package addrs dependency on external module
github.com/hashicorp/terraform-registry-address, which is not ideal but
that one at least has a relatively shallow dependency subgraph, so there's
not so much urgency to tidy that one.
