- Rename doom-module-path -> doom-module-expand-path, to better reflect
its purpose.
- Optimize doom-module-locate-path to try caches and
locate-file-internal, before looping through doom-modules-dirs.
- Rely on file-name-concat to join paths, rather than string
concatenation. file-name-concat is more robust for the purpose and
has lower overhead than expand-file-name.