test/goroot: cover loader frontend diagnostics

[cpunion]

Summary

• cover 5 Go 1.26 loader/import/embed errorcheck cases and retire their xfails
• keep pure go/types versus cmd/compile/types2 wording aliases in the GOROOT test harness:
  • non-canonical import-path punctuation
  • cross-package field/method quoting and nearby-name suggestions
• keep product-code changes limited to diagnostics that reflect real compiler behavior:
  • reject absolute import paths instead of leaking the loader's no metadata detail
  • preserve cmd/compile's semantic-error precedence for local go:embed variables and pre-Go-1.16 language mode when go list validates the trailing pattern first

The loader no longer rewrites general go/types or driver diagnostics. Ordinary malformed imports, field diagnostics outside GOROOT errorcheck matching, and current-version embed pattern failures retain their native llgo/go/packages output.

Cases

• fixedbugs/issue11362.go: GOROOT-only wording alias
• fixedbugs/issue31053.dir/main.go: GOROOT-only wording aliases
• import6.go: loader absolute-import validation
• embedvers.go: loader/compiler diagnostic precedence
• embedfunc.go: loader/compiler diagnostic precedence

Validation

• Rebased onto current main at ce7611414, including merged test/goroot: normalize go/types diagnostics #2104, test/goroot: match exact parser recovery diagnostics #2105, ci: pin Go 1.26 across CI and release pipelines #2108, [Based on #2108] ci: focus compatibility matrix on Go 1.24 and 1.26 #2110, and build: add initial -ldflags -s and -w support #2113; all adjacent xfail removals are retained.
• Darwin/arm64, Go 1.26.5:
  • complete internal/packages tests pass
  • complete test/goroot package tests pass
  • TestToolCompileFrontendDiagnosticNormalization passes with -vet=off
  • all 5 focused GOROOT cases pass with a 3 GiB per-process RSS guard
• Every product helper added or adjusted by this change has 100% statement coverage; internal/packages package coverage is 72.6%.
• git diff --check passes.

The default Go 1.26.5 vet invocation still panics in its vendored x/tools/refactor/satisfy on unchanged main; the affected integration tests therefore use -vet=off.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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