lib: provide a correct implementation for parsing semvar tags

[shoenig]

A few bits of code chop strings up to get at version strings, implement this correctly in the library so we can reuse the code

[billsmith]

I'll look into using hashicorp/go-version for this.

[shoenig]

There's a rudimentary version here:
https://github.com/modprox/taggit/blob/master/tag.go#L10

however it doesn't capture the "extensions" (? i dunno what else to call them) that semver annoyingly supports. See sections 9 and 10 of the semver guidelines: https://semver.org/
If hashicorp/go-version supports that stuff, great! Otherwise we do need to take those versioning things into account, for correct module version resolution in the proxy.

[billsmith]

Judging by the unit tests, sections 9 and 10 are covered: https://github.com/hashicorp/go-version/blob/master/version_test.go

[billsmith]

Sorry, I won't have time to work on this any time soon.