Fixes #2480
This fixes the problem where in some occasions dependencies from workspace are not loaded correctly when --path is passed. For example, buf build workspacedir/a --path workspacedir/a/foo.proto when workspacedir/b and workspacedir/c also exist. In module_file_set_builder.go module b and module c would be hashed based on their TargetFileInfos, which are both empty slices due to the --path filter. Same hash means only one of them gets added to the list of potential dependencies.

In this PR, workspace modules are distinguished by their directories relative to the workspace directory. This is accomplished by adding WorkspaceDirectory to interface bufmodule.Module.