[bug#69827] Fixing the go importer so that it can import modules located in a monorepo

[Christina O'Donnell]

Hi,

These three patches above allow the go importer to automatically import 
modules that are located in a monorepo where the submodules are 
versioned independently. See https://go.dev/ref/mod#vcs-version:

> If a module is defined in a subdirectory within the repository, that 
is, the module subdirectory portion of the module path is not empty, 
then each tag name must be prefixed with the module subdirectory, 
followed by a slash. For example, the module golang.org/x/tools/gopls is 
defined in the gopls subdirectory of the repository with root path 
golang.org/x/tools. The version v0.4.0 of that module must have the tag 
named gopls/v0.4.0 in that repository.

After this change, I am able to import github.com/gohugio/hugo and its 
400 odd dependencies with much fewer failures.

I'll submit those patches separately once I've tested them.

Kind regards,

Christina

PS. This is my first time sending a patch series to debbugs, so I almost 
certainly did something wrong. Feedback on that would be appreciated.