java.project.addToSourcePath command should be allowed in gradle/maven project

[Poytr1]

This issue is related to #859

https://github.com/eclipse/eclipse.jdt.ls/blob/ca81aa7b935b52c0aff7624fa9f330c832578233/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/commands/BuildPathCommand.java#L45

Currently jdt-langserver doesn't allow users to add folder to source path if the project is a Maven/Gradle project which doesn't make sense.

When I open a source file which isn't included in the build path, there isn't any code intelligence at all. Meanwhile I need to modify the build script to add these source folders.

It will be nice if we have code intelligence on these files(even without auto-build) by adding the folder to source path, or can jdt-langserver just treats them as standalone files?

screenshot:

[fbricon]

For Maven and Gradle project, the "truth" comes from the build descriptor. So adding new source folders from the IDE would likely introduce an impedance mismatch with CLI builds, if those folders are not defined as well. So I'd rather prefer we fixed potential issues with jdt.ls not adding source folders when it should, rather than introduce than providing more ways to users to shoot them in the foot

[rgrunber]

Closing as the discussion can continue on #1954 .