Improve record getter handling

[supersaiyansubtlety]

based on #339 to avoid having to work around SearchMappingsTest rebased

Improves how RecordIndexingVisitor finds record getters:

• only methods with the required descriptor, legal access, and legal names (not toString, etc.) are considered getter candidates
• if only one getter candidate is found, it's always chosen and considered a "definite" getter; this allows it to find getter overrides with non-default instructions in simple cases; before getters would not be found if they had non-default instructions
• if multiple getter candidates are found, one is only selected if it has default getter instructions (as before) and its name matches the component field; this a "probable getter"; this is more strict than before and results in not finding a getter with a mismatched name that was previously found in a test; we can't actually be certain it's the getter, and this behavior matches decompilers'
• if no getter candidates are found, an exception is thrown

Additionally, this exposes methods differentiating between "definite" and "probable" getters in RecordIndexingService because #333 will disallow directly naming definite getters (this PR was extracted from #333)

[supersaiyansubtlety]

ready!

[ix0rai]

looks great! i left a bunch of suggestions to make the javadoc easier to read