Go: fix database inconsistency when receiver has alias type

[owen-mc]

It was reported that codeql/go/ql/test/library-tests/semmle/go/aliases/InterfaceImpls fails DB-CHECK if you add --check-databases to the test target of codeql/go/Makefile. This turns out to be because that test has a method defined on a receiver which is an alias Impl6Alias of the type Impl6. This is equivalent to defining the method on Impl6 directly. There is some code in the extractor which makes sure that objects which are receiver variables get a special label which is the same when seen from different files. That code was not unaliasing the type of the receiver. This PR fixes that, which fixes the db inconsistency.

I do not think this needs any tests. I also don't think it needs a change note.

I also included a minor drive-by refactor. We were defining our own wrapper around types.Unalias when we don't need to (because it already has the property we want, i.e. being idempotent for non-alias types).

[Copilot]

Pull Request Overview

This PR ensures that alias types are unaliased before looking up or labeling methods and types, fixing a database inconsistency when a method receiver uses an alias.

• Unalias receiver types in findMethodWithGivenReceiver so methods on alias types resolve correctly.
• Remove resolveTypeAlias helper and replace its uses with types.Unalias in extractType and getTypeLabel.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go/extractor/trap/labels.go	Unalias the receiver’s type before searching for matching methods.
go/extractor/extractor.go	Inline alias resolution by replacing resolveTypeAlias with types.Unalias in type extraction and labeling.

Comments suppressed due to low confidence (3)

go/extractor/trap/labels.go:172

• Add a unit test for findMethodWithGivenReceiver that defines a method on an alias receiver (e.g., Impl6Alias) and verifies the method is found correctly to prevent future regressions.

unaliasedType := types.Unalias(object.Type())

go/extractor/extractor.go:1588

• Add tests for extractType to confirm alias types are unaliased before labeling, ensuring no alias-related inconsistencies in the database.

tp = types.Unalias(tp)

go/extractor/extractor.go:1766

• Add tests for getTypeLabel to verify that types supplied via types.Unalias produce consistent labels for both aliased and unaliased inputs.

tp = types.Unalias(tp)