Improve Mill 1 support - POC

[lefou]

For Mill 1.x tests, don't generate a plugins.sc but instead generate a meta-build.

This is WIP and comes without a integration test, since I need something rather quick.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.75%. Comparing base (11ab233) to head (a590022).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #232   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files           5        5           
  Lines          48       48           
  Branches        2        2           
=======================================
  Hits           45       45           
  Misses          3        3           

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