This is an adaptation of the installation code from configs/configload,
now using the "earlyconfig" package instead of the "configs" package.
Module installation is an initialization-only process, with all other
commands assuming an already-initialized directory. Having it here can
therefore simplify the API of configs/configload, which can now focus only
on the problem of loading modules that have already been installed.
The old installer code in configs/configload is still in place for now
because the caller in "terraform init" isn't yet updated to use this.
"terraform init" is quite a complex beast in relation to other commands
since almost everything it does is unique to it and thus not factored out
into other packages.
To get some of that sprawl out of the "command" package, this new internal
package will give us somewhere to put this init functionality that is
also useful for test code that needs to mimic the initialization behavior
against fixture directories.
This is an alternative to the full config loader in the "configs" package
that is good for "early" use-cases like "terraform init", where we want
to find out what our dependencies are without getting tripped up on any
other errors that might be present.
The main significant change here is that the package name for the proto
definition is "tfplugin5", which is important because this name is part
of the wire protocol for references to types defined in our package.
Along with that, we also move the generated package into "internal" to
make it explicit that importing the generated Go package from elsewhere is
not the right approach for externally-implemented SDKs, which should
instead vendor the proto definition they are using and generate their
own stubs to ensure that the wire protocol is the only hard dependency
between Terraform Core and plugins.
After this is merged, any provider binaries built against our
helper/schema package will need to be rebuilt so that they use the new
"tfplugin5" package name instead of "proto".
In a future commit we will include more elaborate and organized
documentation on how an external codebase might make use of our RPC
interface definition to implement an SDK, but the primary concern here
is to ensure we have the right wire package name before release.
Martin Atkins