Introduce tfexec.Get() for downloading modules

[radeksimko]

Generally it is possible and probably common to use the init command to initialize a Terraform module, which will install providers, setup remote state backend and download modules.

Sometimes it may be desirable to avoid installing providers, or initializing backends. I have one particular use case in tests for Terraform LS where I'd like to create a "mock module manifest" so that I can test some other functionality which relies on that module manifest, without having to copy around that module manifest statically. i.e. I'd like to generate "test data" programmatically via tfexec.Get(), but then mock out the rest of terraform commands.

I can't think of too many other use cases at this point, but the get command doesn't seem to be going anywhere based on docs at least, so I think it's worth supporting it.