This allows the LV2 source distribution to be used as an include path for
compilers and an LV2_PATH for applications, at the expense of self-contained
bundles.  That's a nice idea, but it made LV2 itself weird and annoying to
depend on.  This rearranges things so that directories in the source tree
correspond more closely to installation directories.

To make this possible, the "aux" directory in the documentation output has been
changed to "style", to avoid the reserved name "aux" on Windows.

Switches to safer and more modern idioms, and addresses many issues raised by
flake8 and pylint.

Python 2 hit EOL long ago, and some systems don't have a "python" at all.