Refactoring PegasusPlugin.
File Count checks, previously part of compatibility checking, are now refactored out into their own tasks.
Added the flag -Prest.idl.skipCheck so users can avoid all IDL compatibility checks.
This functionality was NOT, and still is NOT covered by -Prest.idl.compatibility=OFF, since previously
idl count checks were run during snapshot compatibility checking.

RB=202295
R=gogenc,jbetz,kparikh,si-dev,kjin
A=gogenc,kjin