Moderately heavy reorganization of pyclbr to fix package-related bugs.

Moderately heavy reorganization of pyclbr to fix package-related bugs.

- The _modules cache now uses the full module name.

- The meaning of the (internal!!!) inpackage argument is changed: it
  now is the parent package name, or None.  readmodule() doesn't
  support this argument any more.

- The meaning of the path argument is changed: when inpackage is set,
  the module *must* be found in this path (as is the case for the real
  package search).

- Miscellaneous cleanup, e.g. fixed __all__, changed some comments and
  doc strings, etc.

- Adapted the unit tests to the new semantics (nothing much changed,
  really).  Added some debugging code to the unit tests that print
  helpful extra info to stderr when a test fails (interpreting the
  test failures turned out to be hard without these).