[core] Use ruleset loader in pmd 7

[oowekyala]

Describe the PR

Followup on #2635.

• Use RuleSetLoader everywhere and make RuleSetFactory package-private
• Refactor RuleSetFactoryCompatibility so that it only does replacements in rule reference attributes, and not the whole file. Make it package private too
• Remove many APIs deprecated in [core] New RuleSet API and deprecations for PMD's entry point APIs #2635, including RuleSetsFactoryUtils, and RuleSetNotFoundException

Related issues

• [core] New RuleSet API and deprecations for PMD's entry point APIs #2635
• Fix [core] RulesetFactoryCompatibility stores the whole ruleset file in memory as a string #1451

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by travis)
• Added (in-code) documentation (if needed)

[pmd-test]

	2 Messages
📖	Compared to pmd/7.0.x: This changeset changes 0 violations, introduces 0 new violations, 0 new errors and 0 new configuration errors, removes 0 violations, 1 errors and 0 configuration errors. Full report
📖	Compared to master: This changeset changes 1606 violations, introduces 4187 new violations, 1 new errors and 0 new configuration errors, removes 6137 violations, 10 errors and 2 configuration errors. Full report
✅	Compared to pmd/7.0.x: This changeset changes violations, introduces new violations, new errors and new configuration errors, removes violations, errors and configuration errors. Full report
✅	Compared to master: This changeset changes violations, introduces new violations, new errors and new configuration errors, removes violations, errors and configuration errors. Full report

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