var-naming: improve package naming reporting

[alexandear]

Consider the following improvements to the var-naming rule:

• Generate a single failure per package (instead of one for each file in a package). var-naming: detect meaningless package names #1312 (review)
• Report bad package prefixes (e.g., jsonutils), see var-naming: detect meaningless package names #1312 (comment)
• Add an option to exclude specific bad package names from reporting (e.g., ignoreBadPackageNames), see var-naming: detect meaningless package names #1312 (comment)

[chavacava]

I'll work on generating a single failure per package