Handle abstract code starting with a non-file attribute

[filmor]

I'm not really sure how to provoke this, but I had a file that compiled in OTP24 via rebar3 to abstract code that started with a -compile({no_warn_unused_record, [contract]})., so the first entry of the AbstractCode list was {attribute,0,compile,{nowarn_unused_record,[[contract]]}}. This PR filters the AbstractCode list for -file attributes before matching. I can make this more efficient if you want.

[filmor]

The failing shell test is related to the currently broken rebar_mix, I think.

[ferd]

Yeah the code itself ought to be safe. I do similar stripping in the EPP-adjacent code, though I'm just surprised someone did manage to add attributes before -file, since the -module attribute directly translates to that and is supposed to be first.

Anyway, the code doesn't harm anything else (might slow down processing a bit on super large modules with lots of constructs), and if there's a regression we can probably replace it with a dropwhile + hd call