Support having zero distros defined by a URI config

This is useful for testing distros using forced_requirements(-r distro_a)

hab/parsers/flat_config.py

@@ -197,8 +197,14 @@ def freeze(self):
 
     @hab_property(verbosity=1)
     def versions(self):
-        if self.distros is NotSet:
-            return []
+        distros = self.distros
+        if distros is NotSet:
+            if self.resolver.forced_requirements:
+                distros = {}
+            else:
+                return []
+        if distros == []:
+            distros = {}
 
         # Lazily load the contents of versions the first time it's called
         if "versions" not in self.frozen_data:
@@ -207,7 +213,7 @@ def versions(self):
                 self._alias_mods = {}
             self.frozen_data["versions"] = versions
 
-            reqs = self.resolver.resolve_requirements(self.distros)
+            reqs = self.resolver.resolve_requirements(distros)
             for req in reqs.values():
                 version = self.resolver.find_distro(req)
                 versions.append(version)

tests/test_resolver.py

@@ -486,10 +486,14 @@ def test_resolve_requirements_markers(resolver, platform, marker):
 
 
 @pytest.mark.parametrize(
-    "forced,check",
+    "forced,check,check_versions",
     (
         # No forced items
-        (None, ["the_dcc_plugin_a", "the_dcc_plugin_d", "the_dcc_plugin_e<1.0,<2.0"]),
+        (
+            None,
+            ["the_dcc_plugin_a", "the_dcc_plugin_d", "the_dcc_plugin_e<1.0,<2.0"],
+            [],
+        ),
         # Force
         (
             {
@@ -504,12 +508,13 @@ def test_resolve_requirements_markers(resolver, platform, marker):
                 "the_dcc_plugin_d",
                 "the_dcc_plugin_e==1.1",
             ],
+            ["the_dcc_plugin_c==1.1", "the_dcc_plugin_e==1.1"],
         ),
     ),
 )
-def test_forced_requirements(resolver, helpers, forced, check):
+def test_forced_requirements(resolver, helpers, forced, check, check_versions):
     requirements = {
-        # plugin_a adds an extra dependency outside of the requiremets or forced
+        # plugin_a adds an extra dependency outside of the requirements or forced
         "the_dcc_plugin_a": Requirement("the_dcc_plugin_a"),
         "the_dcc_plugin_e": Requirement("the_dcc_plugin_e<1.0"),
     }
@@ -522,6 +527,13 @@ def test_forced_requirements(resolver, helpers, forced, check):
     resolved = resolver_forced.resolve_requirements(requirements)
     helpers.assert_requirements_equal(resolved, check)
 
+    # Check that forced_requirements work if the config defines zero distros
+    cfg = resolver_forced.resolve("not_set/no_distros")
+    versions = cfg.versions
+    assert len(versions) == len(check_versions)
+    for i, v in enumerate(versions):
+        assert v.name == check_versions[i]
+
 
 @pytest.mark.parametrize(
     "value,check",