This conveniently pointed out potential issues in the LV2 documentation, but
makes the tool very noisy to use on anything else.  Referring to some external
thing really shouldn't be treated like a warning in linked data.  Preventing
mistakes (where full URIs are in the output instead of the expected prefixed
names) will have to be done in some other, better way (probably based on an
explicitly given set of prefixes that should be documented).

The RDFa in specification pages doesn't actually use prefixed names for the
documented resources.  So, simplify things by trimming the namespaces down to a
consistent set that is actually used in the LV2 universe.

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.

Unfortunately, GCC and clang-tidy have conflicting warnings here.  Side with
compiler warnings, being generally more relevant than a tool only used by
developers.