Hide private symbols in Python package.

I realized that the way I structured the pip package
in version 2.0.15 made the private symbols (those whose
names begin with an underscore) to be visible by the importer.
I reworked the package structure so this no longer happens.

This is now fixed in:
https://pypi.org/project/astronomy-engine/2.0.16/

source/python/.gitignore

@@ -1,4 +1,3 @@
 *.pyc
 astronomy_engine.egg-info/
 dist/
-astronomy/__init__.py