modules loaded only from an exe-adjacent modules/ dir or ~/.config, so a
bare `go install`ed binary shipped zero modules. embed the modules/ tree
and fall back to it when no filesystem builtin dir is found.
the on-disk builtin dir and the user-override dir still take precedence,
so a dev tree and a release that ships the folder alongside the binary
keep working unchanged. the embed directive lives at the repo root
because go:embed cannot reach a parent directory, and it registers the
filesystem with the loader through a package hook to avoid an import
cycle back into the root package.
