Implement coremod detection fixer, and an AT remapper

[TheCurle]

See RetroGradle#1 for details, but the short version:

• FML uses a system property called fml.coreMods.load to discover coremods, which used to be filled by FG2.
  This is now filled by LegacyDev.
• FML+FG2 uses a FMLAT property in the jar manifest to look for AT files. This searches for those files and adds them manually.
• FML requires mappings files to do runtime remapping, this used to be set by FG2, so we use the Legacy Extension of RetroGradle's ForgeGradle (RetroGradle/ForgeGradle@c1c537d) to put these mappings in a usable place.

This therefore requires aforementioned ForgeGradle extensions to be useful.

This fixes:

• Mod coremods not being discovered
• Mod ATs not being remapped or discovered properly

[pupnewfster]

Correct me if I am wrong but I don't think this or any of the added files have the second line of the license match the change to the LICENSE-header.txt

[LexManos]

If we're changing the header, we should just move to the simplified header like we have for the rest of the projects.