Provide go/analysis analyzer instance

[dtcaciuc]

This PR partially addresses #165, i.e. it provides an Analyzer but does not rework the internals around it yet.

[dtcaciuc]

Existing tests were copied and are largely unchanged. They are adapted to syntax required by the testing utility package https://pkg.go.dev/golang.org/x/tools@v0.1.0/go/analysis/analysistest. Separate test cases were added to test individual enabled flags.

[echlebek]

Just curious, why the package-scoped variable, rather than a way to produce an analyzer with a function?

[dtcaciuc]

This seems to be the suggested way to organize things. See https://pkg.go.dev/golang.org/x/tools/go/analysis@v0.1.0#hdr-Analyzer

[echlebek]

Should we merge this as is, and make another issue to migrate the internals?

[dtcaciuc]

Not sure if it constitutes migration, but it should be possible to simplify the code to record raw token.Pos rather than immediately resolve it into file & line with addErrorAtPosition() whenever we find an error as its done now. Then after the visitor is done running, either feed that directly into go/analysis function output or convert to file/line and write to errcheck program output.

I'll take the PR out of draft.