Improve test coverage for Go modules

**Is your feature request related to a problem? Please describe.**
The Go mod analyzer has been broken in all kinds of ways and entirely unusable at least since 6.x came out. It seems like most issues go undetected simply because there's no test coverage for basic use cases.

**Describe the solution you'd like**
Improve test coverage; implement tests for basic Go mod analysis use cases.

**Describe alternatives you've considered**

**Additional context**
I'd be happy to look into this myself if no one else is available.