(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

sandbox/grist/main.py

@@ -11,14 +11,15 @@ import functools
 
 import six
 
-from acl_formula import parse_acl_formula
 import actions
-from sandbox import get_default_sandbox
 import engine
 import migrations
 import schema
 import useractions
 import objtypes
+from acl_formula import parse_acl_formula
+from sandbox import get_default_sandbox
+from imports.register import register_import_parsers
 
 import logger
 log = logger.Logger(__name__, logger.INFO)
@@ -107,6 +108,8 @@ def run(sandbox):
   export(eng.load_empty)
   export(eng.load_done)
 
+  register_import_parsers(sandbox)
+
   sandbox.run()
 
 def main():