Coremod improvements, debugging, obf task fixes

[serenibyss]

• Fix runObfClient/runObfServer crash with fml.coreMods.load cli arg passed unnecessarily
• Fix runObfServer crash with pointing to 1.7.10's ServerLaunch class instead of 1.12.2's (fixed in Fix runObfServer main class reference for 1.12.2 GTNewHorizons/RetroFuturaGradle#42, released in RFG 1.3.22)
• Move to updated LegacyLauncher for better logging and coremod debug
• Add prompt for online-mode for first run of runServer (added in Promt user for online-mode when first running runServer GTNewHorizons/RetroFuturaGradle#43, released in RFG 1.3.23)
• Remove mixin and its transitive deps from compile classpath if only forceEnableMixins is enabled (and not usesMixins)
• Fix deprecation warnings

Main feature:

• Add enableCoreModDebug option (with env var substitute CORE_MOD_DEBUG)
  • When enabled, outputs loaded classes to run/CLASSLOADER_TEMP
  • Outputs pretransform raw class and all transformed classes. Usually this just is MCP transformer, but will also have separate output classes for each Mixin or ASM transformer targeting a class
  • Allows for easy diffing between different transformation points to see exactly how your Mixin or ASM affected a class