fix(go): credit package-level test coverage instead of name-pairing only

[aliscott]

Problem

Go's test-coverage mapping only pairs test files to production files by name (foo_test.go -> foo.go). But Go compiles a directory as one package and go test reports coverage package-wide: a test file can exercise any identifier in its package regardless of which file defines it.

On a real Go repo this produced a cluster of false untested_module findings: internal/ingest/ingest_test.go (a 1,000-line suite) covers plan.go (91% by go tool cover), verify.go (100%), match.go (90%) - yet all were flagged as untested because no file named plan_test.go exists. Test health scored 38% on a package with 90 passing tests.

desloppify scan --path .   # Go repo, package-level test suites
# -> 23 untested_module findings, most false

A blind-review subagent independently flagged the same thing: "the detector flags files like plan.go, verify.go, match.go as 'untested modules' because they lack matching *_test.go filenames, but Go package-level tests cover them well".

Fix

• Add an optional plural map_test_to_sources(test_path, production_set) -> set[str] language hook, consumed by naming_based_mapping and get_test_files_for_prod. Languages that don't define it are unaffected.
• Implement it for Go: a _test.go file maps to every production .go file in its directory. This mirrors the language's own semantics - both in-package and external (package foo_test) test files compile and run against exactly that package, and go test -cover attributes coverage package-wide.

Verified against the same Go repo: false positives drop from 25 to 11 findings (the remainder are genuinely untested packages), test health 43.8% -> 81.1%.

Tests added:

• tests/lang/go/test_go_test_coverage.py: package-wide mapping, non-test files ignored
• tests/detectors/coverage/test_test_coverage_mapping_import_and_logic.py: engine credits whole Go package; absent hook is a no-op for other languages

pytest desloppify/tests/lang/go/ desloppify/tests/detectors/coverage/ - 200 passed. Full suite: 2916 passed; the 3 test_bash_unused_imports.py failures also occur on a clean checkout in this environment (macOS bash) and are unrelated.