It is now possible to call rebar as:
rebar xref skip_app=Mod1,Mod2,...
This makes it easy to skip running xref on (e.g)
imported dependencies in your application.
The function rebar_utils:is_skipped_app/0 is added so
that other rebar commands may use it.
For parameterized modules, the beam code will have a compiler
generated new/1 and instance/1 function.
If while checking the beam, xref detects one of those is unused, the
rebars xref wrapper will try to find the location of the definition of
that function in the source code for the module (to give a more
specific warning to the user). Since the function was generated by the
compiler it does not actually exist in the source, and rebar crashes
at that stage. This patch works around that issue.
