Use load-extension-global instead of load-extension

Rename libguile- to libguile-gnuradio- to match guile library naming convention.

The stuff in gnuradio-core ends up with funky names, but that could
be fixed by renaming gnuradio_core_filter.i -> core_filter.i etc.

Major Makefile.am housecleaning. Passes distcheck.

Move all occurrences of swig_built_sources out of Makefile.am's.
Move all SWIG related use of BUILT_SOURCES out of Makefile.am's.
Clean up 'if PYTHON' conditionalization in gr-*

Still left to do: fix Makefile.swig CLEANFILES and no_dist_files...

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