Fix bug when system perl is used. (#11034)

* Fix bug when system perl is used.

If system perl is being used through packages.yaml there cannot be dependent modules.
The  setup_dependent_packages function should not do anything in that case.

var/spack/repos/builtin/packages/perl/package.py

@@ -190,15 +190,19 @@ def setup_dependent_package(self, module, dependent_spec):
            perl('Makefile.PL','INSTALL_BASE=%s' % self.prefix)
         """
 
-        # perl extension builds can have a global perl executable function
-        module.perl = self.spec['perl'].command
+        # If system perl is used through packages.yaml
+        # there cannot be extensions.
+        if dependent_spec.package.is_extension:
 
-        # Add variables for library directory
-        module.perl_lib_dir = dependent_spec.prefix.lib.perl5
+            # perl extension builds can have a global perl
+            # executable function
+            module.perl = self.spec['perl'].command
 
-        # Make the site packages directory for extensions,
-        # if it does not exist already.
-        if dependent_spec.package.is_extension:
+            # Add variables for library directory
+            module.perl_lib_dir = dependent_spec.prefix.lib.perl5
+
+            # Make the site packages directory for extensions,
+            # if it does not exist already.
             mkdirp(module.perl_lib_dir)
 
     @run_after('install')