Support for Monorepos

Hey mate, great tool seems very helpful!
Just wondering if there's a way to use modver with monorepos? I'm having some issues.
We have a repo with a number of different modules in subfolders.

Structure looks something like:
> .git/
> module1/
> go.mod
> go.sum
> module2/
> go.mod
> go.sum

running `modver -git ../.git HEAD~1 HEAD` in the folder module1 gives `Error in comparing: error(s) loading package ./...: -: pattern ./...: directory prefix . does not contain main module or its selected dependencies`

running `modver -git .git HEAD~1 HEAD` in the root folder gives `Error in comparing: error(s) loading package ./...: -: pattern ./...: directory prefix . does not contain main module or its selected dependencies`

It appears we can't run modver unless the module is declared in the top level of the repo, is this correct? Is there any workaround, and if not, could it be possible to add support for this?

Thank you!

Reproduce the reported `modver -git ../.git HEAD~1 HEAD` and `modver -git .git HEAD~1 HEAD` commands against the shown multi-module layout. Start with the package-loading path that emits the `./...` error; done means module subfolders can be compared or the supported workaround is documented.

Written by the indexing model from the issue text.