Remove the common sourceset and restore original build.gradle as suggested

build.gradle

@@ -37,26 +37,12 @@ minecraft {
 
 sourceSets {
     hdskins {
-        java {
-            srcDirs "src/hdskins/java", "src/common/java"
-        }
-        resources {
-            srcDirs "src/hdskins/resources"
-        }
-        refMap = 'hdskins.mixin.refmap.json'
-        
         compileClasspath += main.compileClasspath
+        ext.refMap = 'hdskins.mixin.refmap.json'
     }
     main {
-        java {
-            srcDirs "src/main/java", "src/common/java"
-        }
-        resources {
-            srcDirs "src/main/resources"
-        }
-        refMap = 'minelp.mixin.refmap.json'
-        
         compileClasspath += hdskins.output
+        ext.refMap = 'minelp.mixin.refmap.json'
     }
 }