Move mousebender files to the _vendored/ directory

[agriyakhetarpal]

Description

This PR moves the mousebender adaptation to the vendored/ directory, as a follow-up PR to #178, as requested in #178 (comment).

[agriyakhetarpal]

Should micropip/_vendored/mousebender/ contain an __init__.py file? My opinion is that it shouldn't, as we won't expect anyone to use import micropip._vendored.mousebender in their scripts, but we had micropip/externals/__init__.py so it would have been possible to use import micropip.externals prior to this PR.

I checked https://github.com/search?q=micropip.externals&type=code and the results are either not relevant or are really old, from forks that have not been updated (when micropip existed in the Pyodide repository and we included micropip.externals._pip).

[hoodmane]

Thanks!

[agriyakhetarpal]

Thanks for the review!

[ryanking13]

Thanks Agriya!