Fix preference for X.Y version when mixed with X.Y.Z versions (#4922)

For packages which contain a mix of versions with formats X.Y and
X.Y.Z, if the user entered an X.Y version as a preference in
packages.yaml, Spack would get confused and favor any version A.B.Z
where X=A and Y=B. In the case where there is a mix of these version
types, this commit updates preferences so Spack will favor an exact
match.

lib/spack/spack/package_prefs.py

@@ -112,9 +112,17 @@ def __call__(self, spec):
 
         # integer is the index of the first spec in order that satisfies
         # spec, or it's a number larger than any position in the order.
-        return next(
+        match_index = next(
             (i for i, s in enumerate(spec_order) if spec.satisfies(s)),
             len(spec_order))
+        if match_index < len(spec_order) and spec_order[match_index] == spec:
+            # If this is called with multiple specs that all satisfy the same
+            # minimum index in spec_order, the one which matches that element
+            # of spec_order exactly is considered slightly better. Note
+            # that because this decreases the value by less than 1, it is not
+            # better than a match which occurs at an earlier index.
+            match_index -= 0.5
+        return match_index
 
     @classproperty
     @classmethod

lib/spack/spack/test/concretize_preferences.py

@@ -102,6 +102,11 @@ def test_preferred_versions(self):
         spec = concretize('mpileaks')
         assert spec.version == spack.spec.Version('2.2')
 
+    def test_preferred_versions_mixed_version_types(self):
+        update_packages('mixedversions', 'version', ['2.0'])
+        spec = concretize('mixedversions')
+        assert spec.version == spack.spec.Version('2.0')
+
     def test_preferred_providers(self):
         """Test preferred providers of virtual packages are
         applied correctly