The `runDatagen` task can never be cached by gradle, as it depends on its own outputs

[solonovamax]

The run datagen task can never be cached by gradle, as its inputs will always depend on its outputs.

Reproducing:

1. Create simple fabric mod with data generation enabled
2. Add the following code (kotlin; too lazy to port to groovy. the steps to reproduce still match up without this code, this code just helps understand it)

   tasks {
       val runDatagen by named("runDatagen") {
           doFirst {
               println(inputs.files.files)
           }
       }
   
       processResources {
           doFirst {
               println(inputs.files.files)
               println(outputs.files.files)
           }
       }
   }
   

3. Run the runDatagen task several times without changing anything.

Expected:

All runs after the first are cached.

Actual:

None of the runs are cached.

---

Using the print statements, you can see that the runDatagen task depends on its own outputs in the following manner:

• processResources -> takes /src/main/generated/ as an input
• processResources -> outputs to /build/resources/main/
• runDatagen -> takes /build/resources/main/ as an input
• runDatagen -> outputs to /src/main/generated/

Since processResources is never cached and always re-run, this causes runDatagen to always be re-run. This is quite annoying for people who don't want to check src/main/generated/* into VCS, and instead have their jar task depend on runDatagen (to ensure the generated resources always remain up to date), as it will cause runDatagen to constantly be re-run, even when no changes have been made.