Fix extension packaging with the new pipeline changes.
Review Request #8066 — Created March 20, 2016 and discarded
The new pipeline changes broke any extension-provided .less files that needed to include or reference anything from another module, along with any global variables provided by the app. The arguments to lessc were still being set in the old setting, which Pipeline no longer respects, and it wasn't in a suitable format (it needed to be a list of strings, not a string). This fixes that setting, plus ensures that we don't have broken conflicts between what the app must now provide for normal use and what's needed for packaging. It does this by filtering out app-provided settings, replacing them with ours. It also disables the use of the special Djblets LessCompiler's import script for checking outdated dependencies. This isn't needed during packaging, and in fact fails due to static file storage access control checks.
I was able to package Power Pack. Checked that the compiled .css files
contained what I expected, diffing with ones generated against RB 2.5.
Only differences were the presence of sourcemap indicators and removal
of older prefixed CSS rules.