Make pathmod give absolute paths

This was broken when pathmod was turned into a standalone script.

Bug: 340648588
Test: gomod frameworks-minus-apex from build/soong
Change-Id: Ibd5eaf2c29d232c1a7da432723f7c6b6fa8bce83
diff --git a/bin/pathmod b/bin/pathmod
index 90d84b5..f5560ba 100755
--- a/bin/pathmod
+++ b/bin/pathmod
@@ -31,7 +31,8 @@
         sys.stderr.write("usage: pathmod <module>\n")
         sys.exit(1)
 
-    print(modinfo.GetModule(modinfo.ReadModuleInfo(), argv[1])['path'][0])
+    path = modinfo.GetModule(modinfo.ReadModuleInfo(), argv[1])['path'][0]
+    print(os.path.join(os.getenv("ANDROID_BUILD_TOP", ""), path))
 
 if __name__ == "__main__":
     main(sys.argv)