* stacks: Make path.module and path.root relative, to match documentation
Previously, we were just handing off the absolute path produced by the
sourcebundle struct. But that's no good, because the value doesn't stay
consistent between plan and apply when running under tfc-agent. (It uses working
directories named after the unique job ID.) It also doesn't match the
documentation, which describes these as relative paths. This was preventing
people from uploading module-provided files to create aws lambda functions, for
example.
This commit addresses that by converting the source-bundle provided module path
to a relative path (relative to Terraform's working directory). In tfc-agent for
stacks runs, that ends up being the directory directly above the sourcebundle
directory, and all the paths below that are consistent between plan and apply.
* I think this is how the changelog thing works?
