Stac refactor

Preliminary plan is to change the structure:

- /src/geospatial-tools
  
  - stac/
    - copernicus/
      - constants.py
    - planetary_computer/
      - constants.py
    - _other_source_/
      - constants.py 
    - sentinel_1.py
    - sentinel_2.py
    - sentinel_3.py
    - landsat_8.py
    - landsat_9.py
    - stac_client.py 