Refactors org.python.core.imp

All possbile calls to the top-level module script during import are
now guarded by a module import lock, which is now per PySystemState.

Removes statics for the module import lock and the SyspathLoader from
org.python.core.imp. Instead, make them instances of PySystemState. In
the case of the SyspathLoader (a ClassLoader), this change removes
another source of possible resource leaks.

Fixes http://bugs.jython.org/issue2205