Don't infer lsif-java auto-index config for non-indexable repos

Created by: olafurpg

Previously, we inferred lsif-java indexing configuration for repos that didn't have any Java or Scala source files. This caused indexing jobs to fail when we shouldn't have indexed the repo in the first place. This commit fixes that issue by requiring the repo to contain at least a single Java or Scala file in order to infer the lsif-java indexing configuration.

The refactoring for this commit also made it easy to fix another bug where lsif-java would fail to index repos that contained both a toplevel pom.xml file and lsif-java.json file (because it ambiguous which build tool to use). Now, we use the lsif-java index --build-tool=NAME to explicitly set the build tool to LSIF when there is an lsif-java.json file.