dependencies: Don't Repeat Yourself when it comes to lookup methods

We need to extend the candidates the same way per method, but we handle
each method twice: once in explicit method checks, and once for auto. We
can just handle auto as a special list of methods, though.

mesonbuild/dependencies/detect.py

@@ -192,37 +192,30 @@ def _build_external_dependency_list(name: str, env: 'Environment', for_machine:
 
     candidates: T.List['DependencyGenerator'] = []
 
-    # If it's explicitly requested, use the dub detection method (only)
-    if 'dub' == kwargs.get('method', ''):
-        candidates.append(functools.partial(DubDependency, name, env, kwargs))
-        return candidates
-
-    # If it's explicitly requested, use the pkgconfig detection method (only)
-    if 'pkg-config' == kwargs.get('method', ''):
-        candidates.append(functools.partial(PkgConfigDependency, name, env, kwargs))
-        return candidates
-
-    # If it's explicitly requested, use the CMake detection method (only)
-    if 'cmake' == kwargs.get('method', ''):
-        candidates.append(functools.partial(CMakeDependency, name, env, kwargs))
-        return candidates
+    if kwargs.get('method', 'auto') == 'auto':
+        # Just use the standard detection methods.
+        methods = ['pkg-config', 'extraframework', 'cmake']
+    else:
+        # If it's explicitly requested, use that detection method (only).
+        methods = [kwargs['method']]
 
-    # If it's explicitly requested, use the Extraframework detection method (only)
-    if 'extraframework' == kwargs.get('method', ''):
-        # On OSX, also try framework dependency detector
-        if env.machines[for_machine].is_darwin():
-            candidates.append(functools.partial(ExtraFrameworkDependency, name, env, kwargs))
-        return candidates
+    # Exclusive to when it is explicitly requested
+    if 'dub' in methods:
+        candidates.append(functools.partial(DubDependency, name, env, kwargs))
 
-    # Otherwise, just use the pkgconfig and cmake dependency detector
-    if 'auto' == kwargs.get('method', 'auto'):
+    # Preferred first candidate for auto.
+    if 'pkg-config' in methods:
         candidates.append(functools.partial(PkgConfigDependency, name, env, kwargs))
 
-        # On OSX, also try framework dependency detector
+    # On OSX only, try framework dependency detector.
+    if 'extraframework' in methods:
         if env.machines[for_machine].is_darwin():
             candidates.append(functools.partial(ExtraFrameworkDependency, name, env, kwargs))
 
-        # Only use CMake as a last resort, since it might not work 100% (see #6113)
+    # Only use CMake:
+    # - if it's explicitly requested
+    # - as a last resort, since it might not work 100% (see #6113)
+    if 'cmake' in methods:
         candidates.append(functools.partial(CMakeDependency, name, env, kwargs))
 
     return candidates