Fix our go.mod version string parser regex pattern

[eskultety]

We currently don't allow prerelease version identifiers (e.g. beta/rc, etc.) and commentaries on the version string line (e.g. go 1.21.0//comment.

This PR mainly fixes a nasty bug where we fell back to picking an older Go version because of ^this which resulted in a very strange error trying to build buildah from git:

2024-05-29 15:48:50,010 INFO Fetching the gomod dependencies at subpath buildah
2024-05-29 15:48:50,011 INFO Fetching the gomod dependencies at the "buildah" directory
2024-05-29 15:48:52,913 INFO go.mod reported versions: '1.20'[go], '-'[toolchain] (cachi2 enforced)
2024-05-29 15:48:52,927 INFO Using Go release: go1.20
2024-05-29 15:48:52,928 INFO Vendoring the gomod dependencies
2024-05-29 15:49:17,954 INFO Retrieving the list of packages
2024-05-29 15:49:23,014 ERROR RuntimeError: Package parent module was not found
Traceback (most recent call last):

even though buildah uses 1.21 already. This PR also updates some unit tests and adds debug logs to keep better track of dealing with version string parsing.

Maintainers will complete the following section

• [ ] Commit messages are descriptive enough
• [ ] Code coverage from testing does not decrease and new code is covered
• [ ] Docs updated (if applicable)
• [ ] Docs links in the code are still valid (if docs were updated)

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[eskultety]

Note that commits 5 and 6 are going to be squashed together before merging, I split them only for review purposes to make the regex changes easier to track.

[eskultety]

Since v1:

• added a commit that further tweaks how and when we report what version we parsed out of the go.mod file. I verified that this PR fixes the problem reported in the description based on the original buildah submodule ref used here: https://github.com/konflux-ci/buildah-container/commit/588f5baf3d2e4d8e4ffa02d7ed604c6543874dff

[eskultety]

Note that commits 5 and 6 are going to be squashed together before merging, I split them only for review purposes to make the regex changes easier to track.

As advertised, I squashed the two before merging.