Clean up dist/package.mill

[lihaoyi]

• Renames test-mill-dev -> test-dist-run, test-mill-release -> test-install-local
• Rename InstallModule#jar to InstallModule#executableRaw
• Move def prependShellScript logic into def launcher; previously it was only used there, although it normally would be used by assembly, which was very confusing
• Make def executableRaw directly use assembly with proper forkArgs/forkShellArgs/forkCmdArgs configuration
• Remove def launcherScript and just use Jvm.launcherUniversalScript directly