Fix maturin bundling for rust-python code

[kylebarron]

Change list

• Move Python code into python/ directory. This is suggested by the maturin docs here (See note that this structure is recommended to avoid a common ImportError pitfall).
• Rename internal rust library from pgstacrs to pypgstac.pgstacrs
• Update import

Now this includes the rust binary (see pypgstac/pgstacrs.abi3.so).

> maturin build -o dist
> unzip -l dist/pypgstac-0.7.10.dev0-cp38-abi3-macosx_11_0_arm64.whl
Archive:  dist/pypgstac-0.7.10.dev0-cp38-abi3-macosx_11_0_arm64.whl
  Length      Date    Time    Name
---------  ---------- -----   ----
     2311  08-16-2023 17:40   pypgstac-0.7.10.dev0.dist-info/METADATA
      102  08-16-2023 17:40   pypgstac-0.7.10.dev0.dist-info/WHEEL
       49  08-16-2023 17:40   pypgstac-0.7.10.dev0.dist-info/entry_points.txt
     8931  08-16-2023 17:40   pypgstac/db.py
       42  08-16-2023 17:40   pypgstac/version.py
       92  08-16-2023 17:40   pypgstac/__init__.py
     5601  08-16-2023 17:40   pypgstac/hydration.py
        0  08-16-2023 17:40   pypgstac/py.typed
    25857  08-16-2023 17:40   pypgstac/load.py
     5453  08-16-2023 17:40   pypgstac/migrate.py
     3443  08-16-2023 17:40   pypgstac/pypgstac.py
   889350  08-16-2023 17:40   pypgstac/pgstacrs.abi3.so
     1010  08-16-2023 17:40   pypgstac-0.7.10.dev0.dist-info/RECORD
---------                     -------
   942241                     13 files