Resolve GOPATH and GOMODCACHE through one go_env helper instead of two drifting copies #1834

[agent] Filed by the scheduled architecture audit (audit-core). Register: register.

Kind: refactor. Register ID: A03.

Problem

The crate applies Go's default for "GOMODCACHE, else the first GOPATH entry's pkg/mod, else $HOME/go/pkg/mod" in two places, and the two copies differ:

1. crawlers/go_crawler.rs#L185-L209 get_gomodcache takes the first non-empty GOPATH entry. If GOPATH has only empty entries (for example :), it falls through to fs::home_dir()/go/pkg/mod.
2. utils/go_installed_tools.rs#L176-L186 builds gopaths, which comes out empty in the same case. It then gets user_cache = None and never falls back to the home default. It also re-filters go_env results with .filter(|v| !v.is_empty()), which does nothing because go_env already drops empty values.

So for the same environment, the crawler and the stale-installed-tool audit can disagree about which directory is the module cache.

Proposed change

• Add pub(crate) fn gopath_entries(home: Option<&Path>) -> Vec<PathBuf> and pub(crate) fn gomodcache(home: Option<&Path>) -> Option<PathBuf> to utils/go_env.rs. That is about 20 lines, with one rule for empty entries and the home fallback.
• Use them from both get_gomodcache and go_installed_tools, and delete the redundant empty filters.
• Pick the behavior that matches go env GOMODCACHE for a GOPATH of only empty entries. Go treats it as unset, so fall back to the home default.

Size: 3 files, about +25/−25.

Acceptance criteria

• No change for normal environments.
• New unit tests on the helpers (injected env through go_env_with) cover: GOMODCACHE set, multi-entry GOPATH, GOPATH of only empty entries, and no GOPATH.
• The existing go_crawler and go_installed_tools tests pass.