[mypyc] Make compilation order with multiple files consistent

[p-sawicki]

The mypyc plugin sets all modules inside a group as dependent on each other so they are part of a single SCC. When we compile the SCC, we compile its modules according to the order in scc.mod_ids. mod_ids is a set so the order is non-deterministic.

This might result in random failures when multiple files are compiled together and we have a bug in mypyc caused by relying on data that is dependent on the compilation order. For example if we are compiling two files a.py and b.py each with a single class and an attribute of ClassIR representing the class A that is not set during the preparation phase is used when compiling the class B, the result will depend on whether a.py was compiled before b.py.

To fix, make the order deterministic based on module names by sorting them first. Also sort the source files and groups passed to mypycify so that invoking mypyc a.py b.py and mypyc b.py a.py produces identical results.