#8720: fix inspect regression by teaching getsourcefile about linecache.

#8720: fix inspect regression by teaching getsourcefile about linecache.

The fix for issue 4050 caused a regression:  before that fix, source
lines in the linecache would eventually be found by inspect.  After the
fix inspect reports an error earlier, and the source isn't found.
The fix for the fix is to have getsourcefile look in the linecache for
the file and return the psuedo-filename if the source is there, just as
it already returns it if there is a PEP 302 loader.