Remove the external BS4 dependency

[JarrettR]

In addition to making this plugin difficult to install, it also removes the ability to use the new KiCad plugin manager.

BeautifulSoup can be downloaded and included as a set of flat files in this repo, and in fact, it has been for some time, in the BS folder. That folder has been unused so far, though.

The problem with using it is that Python embedded in KiCad treats relative path imports really strangely. I attempted to get this working last year for a little while, but it was tricky, undocumented, and wasn't necessary for the minimum viable product.

Trying again to figure this out is becoming a higher priority now.