copy non-java files into remapped sources jar (fixes #126 and #118) (#129)

* copy non-java files into remapped sources jar

* Simplify isJavaFile

* remove unused import

* Use path API

src/main/java/net/fabricmc/loom/util/SourceRemapper.java

@@ -135,6 +135,8 @@ public class SourceRemapper {
 			project.getLogger().warn("Could not remap " + source.getName() + " fully!", e);
 		}
 
+		copyNonJavaFiles(srcPath, dstPath, project, source);
+
 		if (dstFs != null) {
 			dstFs.close();
 		}
@@ -142,8 +144,28 @@ public class SourceRemapper {
 		if (isSrcTmp) {
 			Files.walkFileTree(srcPath, new DeletingFileVisitor());
 		}
+
 	}
 
+    private static void copyNonJavaFiles(Path from, Path to, Project project, File source) throws IOException {
+        Files.walk(from).forEach(path -> {
+            Path targetPath = to.resolve(from.relativize(path).toString());
+            if (!isJavaFile(path) && !Files.exists(targetPath)) {
+                try {
+                    Files.copy(path, targetPath);
+                } catch (IOException e) {
+                    project.getLogger().warn("Could not copy non-java sources '" + source.getName() + "' fully!", e);
+                }
+            }
+        });
+    }
+
+    private static boolean isJavaFile(Path path) {
+        String name = path.getFileName().toString();
+        // ".java" is not a valid java file
+        return name.endsWith(".java") && name.length() != 5;
+    }
+
 	public static class TinyReader extends MappingsReader {
 		private final Mappings m;
 		private final String from, to;