Maintenance script to check LESS files for validity

A reworked version of script from I068686854ad79e2f63a08d81b1af02f373110613

Move leccs instantiation code to ResourceLoader because it doesn't depend on any
particular module's state.

Change-Id: I733b53171dca77f50a30e5bd0bd5f1b456e4c85d
Merge "Maintenance script to check LESS files for validity"