import decky_plugin during plugin loading and set it up as module

[Jan200101]

Please tick as appropriate:

• [x] I have tested this code on a steam deck or on a PC
• [x] My changes generate no new errors/warnings
  • This was not tested against the PyInstaller output
• [ ] This is a bugfix/hotfix
• [ ] This is a new feature

Description

Currently the plugin logic relies on decky_plugin.py being on the path, this PR makes it so that decky_plugin is loaded directly and put into the sys modules ready to be used.

Tested against 2 copies of the plugin template with prints added to verify that the imported instance desky_plugin is indeed different.

Had to import from an absolute path because relative imports can only be done in a package

[Jan200101]

completely untested both with and without PyInstaller. Plugin path may be wrong, idk.