Move importlib.abc.SourceLoader to _bootstrap.

Required updating code relying on other modules to switch to _bootstrap's
unique module requirements. This led to the realization that
get_code was being too liberal in its exception catching when calling set_data
by blindly grabbing IOError. Shifted the responsibility of safely ignoring
writes to a read-only path to set_data.

Importlib is still not relying on SourceLoader yet; requires creating a
SourcelessLoader and updating the source finder.