Hey JAXPLE, I'm actually not sure how that part of Fabric works. This might be a better question for the Fabric Discord.
Looking at https://maven.fabricmc.net/net/fabricmc/dev-launch-injector/0.2.1+build.8/, there don't seem to be any source jars for that artifact.
If this happened while starting your IDE or running one of the usual Gradle tasks, I would try running:
./gradlew genSources --refresh-dependencies
This can sometimes fix weird issues with Gradle's cache files.
