synchronize ECJ linter with eclipse IDE [LUCENE-9881]

Currently the IDE configuration is different from the linter.

Some checks in the eclipse linter are currently disabled, but will show in the IDE and vice-versa.

In the eclipse IDE, the experience is terrible: there are hundreds of useless warnings that won't actually fail lucene's build. It also enforces some things as error that make experimentation frustrating (such as unused imports)

I think we should remove the duplication. linter config should have all of the compiler config. And we should just combine linter config, converting all errors into warnings, with the formatting settings into one file during `gradle eclipse`

This way, if you see an eclipse warning, you know it will fail the lucene build. But you are no longer forced to fix unused imports to quickly try something out and run a unit test in the IDE, for example.

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Migrated from [LUCENE-9881](https://issues.apache.org/jira/browse/LUCENE-9881) by Robert Muir (@rmuir), updated Mar 27 2021
Pull requests: https://github.com/apache/lucene/pull/46

Start with the linter, compiler, and formatting configurations used by the `gradle eclipse` entry point, then review pull request 46 for the existing work. Done means the generated Eclipse settings combine these configurations, convert errors to warnings, and align IDE warnings with checks that fail the Lucene build.

Written by the indexing model from the issue text.