Old concretizer: prevent unexpected propagation of external config (#20976)

When using an external package with the old concretizer, all
dependencies of that external package were severed. This was not
performed bidirectionally though, so for an external package W with
a dependency on Z, if some other package Y depended on Z, Z could
still pull properties (e.g. compiler) from W since it was not
severed as a parent dependency.

This performs the severing bidirectionally, and adds tests to
confirm expected behavior when using config from DAG-adjacent
packages during concretization.

lib/spack/spack/spec.py

@@ -2278,6 +2278,8 @@ def _expand_virtual_packages(self, concretizer):
                 # If replacement is external then trim the dependencies
                 if replacement.external:
                     if (spec._dependencies):
+                        for dep in spec.dependencies():
+                            del dep._dependents[spec.name]
                         changed = True
                         spec._dependencies = DependencyMap()
                     replacement._dependencies = DependencyMap()

lib/spack/spack/test/concretize.py

@@ -925,6 +925,35 @@ def test_compiler_constraint_with_external_package(
         assert s.external
         assert s.satisfies(expected)
 
+    @pytest.mark.regression('20976')
+    @pytest.mark.parametrize('compiler,spec_str,expected,xfailold', [
+        ('gcc', 'external-common-python %clang',
+         '%clang ^external-common-openssl%gcc ^external-common-gdbm%clang', False),
+        ('clang', 'external-common-python',
+         '%clang ^external-common-openssl%clang ^external-common-gdbm%clang', True)
+    ])
+    def test_compiler_in_nonbuildable_external_package(
+            self, compiler, spec_str, expected, xfailold
+    ):
+        """Check that the compiler of a non-buildable external package does not
+           spread to other dependencies, unless no other commpiler is specified."""
+        packages_yaml = {
+            'external-common-openssl': {
+                'externals': [
+                    {'spec': 'external-common-openssl@1.1.1i%' + compiler,
+                     'prefix': '/usr'}
+                ],
+                'buildable': False
+            }
+        }
+        spack.config.set('packages', packages_yaml)
+
+        s = Spec(spec_str).concretized()
+        if xfailold and spack.config.get('config:concretizer') == 'original':
+            pytest.xfail('This only works on the ASP-based concretizer')
+        assert s.satisfies(expected)
+        assert 'external-common-perl' not in [d.name for d in s.dependencies()]
+
     def test_external_packages_have_consistent_hash(self):
         if spack.config.get('config:concretizer') == 'original':
             pytest.skip('This tests needs the ASP-based concretizer')