Fix and unify relocation of Python scripts
Link out/share/lilypond/current/python to python/ in the source directory and use it to find custom modules. This makes both
work for in-tree and out-of-tree builds. The logic gives priority to modules in the source directory (see #6015 (closed)).
Moreover, also enable dynamic relocation for installation trees
with a default layout (version number instead of 'current'). This
is handy for
make install DESTDIR=....