Add Go module collection support

[dagger-codex]

Summary

• change Go.modules to return a GoModules collection keyed by discovered module paths
• keep Go.module as the direct lookup entry point for path/find-up behavior
• update e2e expectations at the call sites that previously projected module paths from a list

Dependency

Draft until dagger/dagger#13299, or equivalent Dagger collection support, lands. This intentionally uses the new Dang @keys/@get directives and does not bump engineVersion to a release that does not contain them yet.

Verification

• git diff --check
• (cd helpers/go-includes && go test ./...)
• dagger functions -m . currently reaches module loading and fails only because the installed engine reports @keys/@get as undeclared directives.