dependencies: delay often-unused imports

We expose detect.py as the mesonbuild.dependencies entrypoint and import
it upfront everywhere. But unless the `dependency()` function is
actually invoked, we don't need *any* of the private implementations for
this.

Avoid doing so until, as part of actual dependency lookup, we attempt
that specific dependency method. This avoids importing big modules if
`method:` is specified, and in most cases hopefully pkg-config works and
we can avoid importing the cmake implementation particularly.

Actually avoiding most of these imports requires more refactoring. But
even so, the garden path no longer needs to import the dub dependency
impl.

mesonbuild/dependencies/detect.py

@@ -14,10 +14,6 @@
 from __future__ import annotations
 
 from .base import ExternalDependency, DependencyException, DependencyMethods, NotFoundDependency
-from .cmake import CMakeDependency
-from .dub import DubDependency
-from .framework import ExtraFrameworkDependency
-from .pkgconfig import PkgConfigDependency
 
 from ..mesonlib import listify, MachineChoice, PerMachine
 from .. import mlog
@@ -201,21 +197,25 @@ def _build_external_dependency_list(name: str, env: 'Environment', for_machine:
 
     # Exclusive to when it is explicitly requested
     if 'dub' in methods:
+        from .dub import DubDependency
         candidates.append(functools.partial(DubDependency, name, env, kwargs))
 
     # Preferred first candidate for auto.
     if 'pkg-config' in methods:
+        from .pkgconfig import PkgConfigDependency
         candidates.append(functools.partial(PkgConfigDependency, name, env, kwargs))
 
     # On OSX only, try framework dependency detector.
     if 'extraframework' in methods:
         if env.machines[for_machine].is_darwin():
+            from .framework import ExtraFrameworkDependency
             candidates.append(functools.partial(ExtraFrameworkDependency, name, env, kwargs))
 
     # Only use CMake:
     # - if it's explicitly requested
     # - as a last resort, since it might not work 100% (see #6113)
     if 'cmake' in methods:
+        from .cmake import CMakeDependency
         candidates.append(functools.partial(CMakeDependency, name, env, kwargs))
 
     return candidates

test cases/unit/113 empty project/expected_mods.json

@@ -208,7 +208,6 @@
       "mesonbuild.dependencies.cuda",
       "mesonbuild.dependencies.detect",
       "mesonbuild.dependencies.dev",
-      "mesonbuild.dependencies.dub",
       "mesonbuild.dependencies.factory",
       "mesonbuild.dependencies.framework",
       "mesonbuild.dependencies.hdf5",
@@ -272,6 +271,6 @@
       "mesonbuild.wrap",
       "mesonbuild.wrap.wrap"
     ],
-    "count": 98
+    "count": 97
   }
 }

unittests/platformagnostictests.py

@@ -253,4 +253,4 @@ class PlatformAgnosticTests(BasePlatformTests):
             expected = json.load(f)['meson']['modules']
 
         self.assertEqual(data['modules'], expected)
-        self.assertEqual(data['count'], 98)
+        self.assertEqual(data['count'], 97)