SI-8392 Guards QuasiquoteClass against non-availability in scala-reflect

This assimilates behavior on Quasiquotes to the one on Reflection : if we
don't have the right resource on classpath, we do without rather than
throwing.

See the bug report for more on rationale (-Xsource flag, early quasiquote
loading in Analyzer through FastTrack, etc).

Review by @gkosskakowski or @xeno-by