(core) Move file import plugins into core/sandbox/grist

Summary:
Move all the plugins python code into the main folder with the core code.

Register file importing functions in the same main.py entrypoint as the data engine.

Remove options relating to different entrypoints and code directories. The only remaining plugin-specific option in NSandbox is the import directory/mount, i.e. where files to be parsed are placed.

Test Plan: this

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D2965

plugins/core/manifest.yml

@@ -20,5 +20,4 @@ contributions:
 
 scripts:
   build:
-  # Note that ${XUNIT:+xxx} inserts "xxx" when XUNIT is set, and nothing otherwise.
-  test: $GRIST_PYTHON -m runtests discover -v -s /sandbox ${XUNIT:+--xunit}
+  test: