Don't inject import statements in package recipes

Remove a hack done by RepoLoader, which was injecting an extra
```
from spack.package import *
```
at the beginning of each package.py

share/spack/templates/mock-repository/package.pyt

@@ -1,3 +1,5 @@
+from spack.package import *
+
 class {{ cls_name }}(Package):
     homepage = "http://www.example.com"
     url = "http://www.example.com/root-1.0.tar.gz"