Refactor PackageWarningDefinition into PackageWarning, as an enhanced enum

[srawlins]

Previously, most of the data for a package warning kind was stuffed into a class, PackageWarningDefinition, and a map mapped the PackageWarning enum values into PackageWarningDefinition. But they should be combined into one class (enhanced enum) as they are here. Also:

• packageWarningsByName is removed,
• packageWarningDefinitions is removed,
• missingExampleFile and unknownHtmlFragment both get "flag names" as part of the refactor, so they can be specified at the command line or in an options file. Fixes Expose missingExampleFile and unknownHtmlFragment warnings #3546
• PackageWarning.template, PackageWarning.warnablePrefix, and PackageWarning.referredFromPrefix are all made private.

The diff looks huge, but I did not change any of the values from the two classes that I merged, like long help text, short help text, or flag names.

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[keertip]

Was a bit confusing with all the diffs, your comments helped figure out the actual changes. Thanks for making it clear.

[srawlins]

Yeah that diff was basically useless; sorry about that.