Switch to x/tools/go/analysis package

[NathanBaulch]

I went looking for a more maintainable unit test pattern for linters (as mentioned in #36) and quickly discovered the golang.org/x/tools/go/analysis package which is used by just about every other linter out there. This popular package was introduce a year after prealloc's inception.

The switch touches a lot of code but was pretty straightforward overall.

• All code dealing with discovering and parsing .go files is gone, including all of import.go.
• Unit tests use a comment convention to declare expected lint suggestions inline. Much easier.
• pkg.Hint is replaced by analysis.Diagnostic.
• Additional standardized linter CLI options are added (go/analysis is quite opinionated) for things like json output and test file exclusion.
• Non-zero exit code 3 is always returned when suggestions are found.

I've confirmed that the same results are returned as before, and the unit tests are passing, however there are some differences in how the tool is run. This would be hidden from most users who use prealloc via golangci-lint though.

[alexkohler]

This looks reasonable, thanks again for the PRs/cleanup! 🤝

[NathanBaulch]

I've refactored the current testdata suite into one scenario per func spread across multiple files, and increased test coverage from 71.7% to 85.5% along the way. This makes each scenario self contained, no need to worry about clashing variable names, etc.