Allow a targeted (named rule) enable to override a module disable (#464)

This allows the following configuration to work as a way of disabling
all of the builtin rules except for specific ones:

```
[tool.fixit]
disable = ["fixit.rules"]
enable = ["fixit.rules:UseClsInClassmethod"]
```

Co-authored-by: surge119 <ly.sergio480@gmail.com>

src/fixit/config.py

@@ -207,6 +207,7 @@ def collect_rules(
     """
 
     all_rules: Set[Type[LintRule]] = set()
+    named_enables: Set[Type[LintRule]] = set()
     if debug_reasons is not None:
         disabled_rules = debug_reasons
     else:
@@ -222,10 +223,19 @@ def collect_rules(
             stack.enter_context(append_sys_path(path))
 
         for qualified_rule in config.enable:
-            all_rules |= set(find_rules(qualified_rule))
+            rules = set(find_rules(qualified_rule))
+            if qualified_rule.name:
+                named_enables |= rules
+            all_rules |= rules
 
         for qualified_rule in config.disable:
-            disabled_rules.update({r: "disabled" for r in find_rules(qualified_rule)})
+            disabled_rules.update(
+                {
+                    r: "disabled"
+                    for r in find_rules(qualified_rule)
+                    if r not in named_enables
+                }
+            )
             all_rules -= set(disabled_rules)
 
         if config.tags:

src/fixit/tests/config.py

@@ -476,6 +476,24 @@ def collect_types(cfg: Config) -> List[Type[LintRule]]:
             )
             self.assertListEqual([UseClsInClassmethod], rules)
 
+        with self.subTest("disable builtins"):
+            rules = collect_types(
+                Config(
+                    disable=[QualifiedRule("fixit.rules")],
+                    python_version=None,
+                )
+            )
+            self.assertListEqual([], rules)
+
+        with self.subTest("override broad opt-out"):
+            rules = collect_types(
+                Config(
+                    disable=[QualifiedRule("fixit.rules")],
+                    enable=[QualifiedRule("fixit.rules", "UseClsInClassmethod")],
+                )
+            )
+            self.assertListEqual([UseClsInClassmethod], rules)
+
         with self.subTest("version match"):
             rules = collect_types(
                 Config(