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 @gkossakowski  or @xeno-by