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script for checking outdated dependencies.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.