Remove deprecated backwards-compatibility APIs from v0.5.0

[reidbaker]

In v0.5.0, several APIs were refactored for the new RuleConfigPatch architecture, but we kept backward-compatibility measures to prevent breaking downstream consumers. These should be removed in the next major version release.

APIs to remove:

• Validator constructor: ruleOverrides parameter.
• LintTargetConfig: rules getter.
• Configuration: configuredRules getter.
• ValidationSession: resolvedRules parameter.
• ValidationSession: resolveRulesForPath parameter.
• src/validator.dart: export 'models/validation_result.dart';