* Use `Tap#formula_dir` instead of `Tap#formula_files` to find formula
file to have better performance and avoid caching issue.
* Change the loader logic to search name -> search alias -> search old name.
This is more consistence with what we do when loading core formula
file.
ClosesHomebrew/homebrew#49484.
Signed-off-by: Xu Cheng <xucheng@me.com>