concretizer: dependency conditions cannot hold if package is external

fixes #20736

Before this one line fix we were erroneously deducing
that dependency conditions hold even if a package
was external.

This may result in answer sets that contain imposed
conditions on a node without the node being present
in the DAG, hence #20736.

var/spack/repos/builtin.mock/packages/external-buildable-with-variant/package.py

@@ -11,5 +11,8 @@ class ExternalBuildableWithVariant(Package):
     url = "http://somewhere.com/module-1.0.tar.gz"
 
     version('1.0', '1234567890abcdef1234567890abcdef')
+    version('0.9', '1234567890abcdef1234567890abcdef')
 
     variant('baz', default=False, description='nope')
+
+    depends_on('c@1.0', when='@0.9')