Add --numeric-srg option for using srg names (vs csv) on MCP dirs

Normally, --srg-in on a directory will load joined.srg or
client.srg and server.srg, translated through fields.csv,
methods.csv, and packages.csv, if available. --numeric will
ignore fields.csv and methods.csv, remapping the jar to
the numeric "srg" names instead of descriptive "csv" names.
(packages.csv will still be used if it exists; if you want
flat packaging then specify joined.srg to --srg-in directly.)