I think, now I have finally got the solution!
After I saw the topic "[1.14.4] How add JEI to own mod?", I focused on trying to find out how other mods do this.
The problem is clear: eclipse has found no source code. And I think this is not a bug, there is just no source code.
As far as I know, the faltDir behaves like a regular maven repository. At any mod I have looked (forge too), there is either a [jar name]-sources.jar (it is like the regular jar, but every .class file is the .java file) or the jar not only contains the .class file, but also the .java files. For example the mezz.jei repo at https://dvs1.progwml6.com/files/maven/
Lines from my build.gradle:
// The mappings can be changed at any time, and must be in the following format.
// snapshot_YYYYMMDD Snapshot are built nightly.
// stable_# Stables are built at the discretion of the MCP team.
// Use non-default mappings at your own risk. they may not always work.
// Simply re-run your setup task after changing the mappings to update your workspace.
mappings channel: 'snapshot', version: '20200723-1.16.1'
// makeObfSourceJar = false // an Srg named sources jar is made by default. uncomment this to disable.
// accessTransformer = file('src/main/resources/META-INF/accesstransformer.cfg')
// Default run configurations.
// These can be tweaked, removed, or duplicated as needed.
The is the comment "an Srg named sources jar is made by default". The fact that it is srg named should not matter, because it gets mapped. But the fact that a [jar name]-sources.jar is created is interesting.
If I were right right, I would just have to find this [jar name]-sources.jar.
And is where I need your help. Where is the sources.jar? Or do I have to add makeObfSourceJar = true (maybe the default is not working)