Fixes #32

Fixes: #29

Although efforts were made for backwards compatibility using
`__getattr__` in `pythonjsonlogger/__init__.py`, this only worked if you
were doing `from pythonjsonlogger import jsonlogger`. When importing by
the full name `import pythonjsonlogger.jsonlogger` it it would fail to
locate the module file and thus not be able to produce a module spec
which then of course causes the import to fail.

We get around this by actually having a module that imports the names it
needs from the new locations.

### Test Plan

- Run unit tests