take Java search scope into account

[martinlippert]

The Java tooling for VSCode introduced a search scope a while ago:

redhat-developer/vscode-java#2649
gayanper/vscode-java@a34e5c5
eclipse-jdtls/eclipse.jdt.ls#3252

This basically allows operations like find references or show hierarchy to focus the results on either all the java sources or from sources excluding test cases.

The idea here would be to take this search scope into account as well for symbols, find references, etc. It would mean that we need to get information from the Java extension about the current search scope (and updates about that setting), and take that information into account. It would either require us to take this scope into account when indexing source code or to filter out test folders from results.