TapLoader: improve load logic
TapLoader: improve load logic

* 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.

Closes #49484.

Signed-off-by: Xu Cheng <9a05244150b861e40c78843b801eed71bca54eac@me.com>

filcab/homebrew,filcab/homebrew,filcab/homebrew,filcab/homebrew