python dependency: fix embed handling for system dependency

Only search for and provide linkage to libpython, if the dependency
expects to be linked to it. Fixes overlinking on Linux / macOS when
pkg-config isn't installed and the sysconfig lookup is used instead.

This was correctly handled for pkg-config rather than deferring it until
use, since commit bf832743441a1171518d7a436164c989be679410 -- but that
handling neglected to cover sysconfig dependencies. And sysconfig would
always try to link to libpython, it just respected the dependency
configuration barely enough to allow falling back to "don't link" if
both link_libpython=False and the library wasn't found.

(cherry picked from commit d3148efe4f469412fbd1d7771b36e3082467979d)
[backported to apply pre-code move]

# Conflicts:
#	mesonbuild/dependencies/python.py

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