More adaptive defaults for modcheck CLI

[Slackow]

• Use environment variable INST_DIR directly if available, 100% resistant against spaces in paths
• Change default category to ssg if instance name contains "ssg"
• Get version from mmc-pack.json if possible

[Slackow]

Just also commited:

• remove space collapsing args for --path and --instance
• avoid some errors when trying to read prism/mmc version
• replace printing and exiting with utility method to do both
• print to standard error where appropriate
• printing help function now also exits

[Slackow]

last commit added it so that if unspecified, the category is automatically set to ssg if ssg-only mods are detected, this does mean all the fabric.mod.json's are fetched twice atm, it's probably a good idea to add a cache for that

[tildejustin]

if it matters it matters, but no need to make it complicated if it doesn't