Issue #24485: Function source inspection fails on closures.

The fix for Issue #21217 introduced a regression that caused
`inspect.getsource` to return incorrect results on nested
functions.  The root cause of the regression was due to
switching the implementation to analyze the underlying
bytecode instead of the source code.

This commit switches things back to analyzing the source code
in a more complete way.  The original bug and the regression
are both fixed by the new source code analysis.