Rename "backend" headers

Working on Vulkan clarified what has always been slightly smelly about the
design and organization here: not everything that is API specific is really in
a "backend" (a PuglBackend).  The concrete example is puglGetProcAddress(),
which only makes sense for GL and is actually implemented in the "backend"
files.  Arguably puglGetContext() is also such a thing.

So, rename the headers so they can be the place where API-specific things go in
general, which happens to include a backend most of the time.  The stub is a
bit of an exception to this, but whatever.  The includes look tidier this way.

In place of the old headers are compatibility stubs that just emit a warning
and include the new version, which will be maintained for a while.

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