Now you cannot randomly rename PKGBUILD files and use them with namcap.
 
I have filed a bug here: https://bugs.archlinux.org/task/57193
I created a patch for my needs:
 
diff --git a/namcap.py b/namcap.py
index b62a2fa..aae20c7 100755
--- a/namcap.py
+++ b/namcap.py
@@ -248,7 +248,7 @@ for package in packages:
 
         if os.path.isfile(package) and tarfile.is_tarfile(package):
                 process_realpackage(package, active_modules)
- elif package.endswith('PKGBUILD'):
+ elif package.startswith('PKGBUILD'):
                 process_pkgbuild(package, active_modules)
         else:
                 print("Error: Cannot process %s" % package)
 
But I think it is better to just assume than all non-tar files are pkgbuilds and should be processed accordingly.