Sort script sources to ensure deterministic generation of child module aliases in the build.mill/package.mill file

[lihaoyi]

• The Seq returned from MillBuildRootModule#scriptSources comes from the keys of a Map, meaning it is arbitrary non-deterministic orders
• Causing the resultant child module aliases (generated in build.mill and package.mill files to reference sub-folder package.mill files) to occur in non-deterministic orders
• Resulting in bytecode changes (e.g. varying the bit position of each reference the lazy val initialization bitmap) that causes lots of stuff to unnecessarily invalidate

Tested manually, seems to remove the misbehavior reported in #4112. Managed to reproduce it in integration/invalidation/codesig-subfolder with some tweaks, so I left the repro in place to guard against regression

This still leaves us open to unnecessary invalidations if someone adds a new subfolder/package.mill file in the middle of the list, but adding new package.mills is pretty uncommon so we can probably fix that in a follow up