python.dependency(): Do not stop when first candidate is not found

It has to lookup the dependency with required=False otherwise it raises
an exception when the first candidate (pkg-config) failed.

mesonbuild/modules/python.py

@@ -500,11 +500,12 @@ class PythonInstallation(ExternalProgramHolder):
         if disabled:
             mlog.log('Dependency', mlog.bold('python'), 'skipped: feature', mlog.bold(feature), 'disabled')
         else:
+            new_kwargs = kwargs.copy()
+            new_kwargs['required'] = False
+            methods = process_method_kw({DependencyMethods.PKGCONFIG, DependencyMethods.SYSTEM}, kwargs)
             for d in python_factory(self.interpreter.environment,
                                     MachineChoice.BUILD if kwargs.get('native', False) else MachineChoice.HOST,
-                                    kwargs,
-                                    process_method_kw({DependencyMethods.PKGCONFIG, DependencyMethods.SYSTEM}, kwargs),
-                                    self):
+                                    new_kwargs, methods, self):
                 dep = d()
                 if dep.found():
                     break

test cases/unit/101 python without pkgconfig/meson.build

@@ -0,0 +1,4 @@
+project('python wihtout pkgconfig', 'c')
+
+# This unit test is ran with PKG_CONFIG=notfound
+import('python').find_installation().dependency()

unittests/platformagnostictests.py

@@ -66,3 +66,7 @@ class PlatformAgnosticTests(BasePlatformTests):
         # platlib is allowed, only python.platlib is reserved.
         fname = write_file("option('platlib', type: 'string')")
         interp.process(fname)
+
+    def test_python_dependency_without_pkgconfig(self):
+        testdir = os.path.join(self.unit_test_dir, '101 python without pkgconfig')
+        self.init(testdir, override_envvars={'PKG_CONFIG': 'notfound'})