Add xref support for multi-app projects
Add xref support for multi-app projects

Add xref support for multi-app projects. Instead of only adding the
top-level ebin directory to the xref server use
rebar_state:project_apps/1 to enumerate the applications for the
project and add the ebin directory for each one.

walrusVision/rebar3,tsloughter/rebar3,processone/rebar3,walrusVision/rebar3,djnym/rebar3,norton/rebar3,rebar/rebar3,fabiankrol/rebar3,rebar/rebar3,ferd/rebar3,vladdu/rebar3,jbalint/rebar3,lrascao/rebar3,kellymclaughlin/rebar3,benoitc/rebar3,zugolosian/rebar3,zugolosian/rebar3,linearregression/rebar3,saleyn/rebar3,project-fifo/rebar3,jbalint/rebar3,processone/rebar3,fabiankrol/rebar3,surik/rebar3,surik/rebar3,sile/rebar3,blt/rebar3,linearregression/rebar3,blt/rebar3,benoitc/rebar3,erlang/rebar3,fauxsoup/rebar3,sile/rebar3,project-fifo/rebar3,fauxsoup/rebar3