Depend on a library and override upstream requirements

[jgsogo]

After implementing #4473 (config variable general.error_on_override), the user can turn any non-explicit dependency override into an error, but there is no way in Conan to declare at the same time that a package:

1. depends on a reference
2. wants to override upstream conflicts

Something like the following in a recipe fails with a Duplicated requirement error:

class Lib(ConanFile):
    requires = (("Hello1/0.1@lasote/stable"), 
                ("Hello2/0.1@lasote/stable"), 
                ("Hello0/0.1@lasote/stable", "override"),
                ("Hello0/0.1@lasote/stable")

---

I suggest the following test (taken from #4771, here: conans/test/functional/graph/conflict_diamond_test.py):

class ConflictDiamondTest(unittest.TestCase):
    conanfile = textwrap.dedent("""
        from conans import ConanFile

        class HelloReuseConan(ConanFile):
            name = "%s"
            version = "%s"
            requires = %s
        """)

    def _export(self, name, version, deps=None, export=True):
        deps = ", ".join(['"%s"' % d for d in deps or []]) or '""'
        conanfile = self.conanfile % (name, version, deps)
        files = {CONANFILE: conanfile}
        self.client.save(files, clean_first=True)
        if export:
            self.client.run("export . lasote/stable")

    def setUp(self):
        self.client = TestClient()
        self._export("Hello0", "0.1")
        self._export("Hello0", "0.2")
        self._export("Hello1", "0.1", ["Hello0/0.1@lasote/stable"])
        self._export("Hello2", "0.1", ["Hello0/0.2@lasote/stable"])

....

    def test_override_explicit_and_depend(self):
        """ ...
        """
        with environment_append({'CONAN_ERROR_ON_OVERRIDE': "True"}):
            conanfile = self.conanfile % ("Hello3", "0.1",
                                          '(("Hello1/0.1@lasote/stable"), '
                                          '("Hello2/0.1@lasote/stable"), '
                                          '("Hello0/0.1@lasote/stable", "override"),
                                          '("Hello0/0.1@lasote/stable"),)')
            self.client.save({CONANFILE: conanfile})
            self.client.run("install . --build missing")
            self.assertIn("Hello2/0.1@lasote/stable requirement Hello0/0.2@lasote/stable overridden"
                          " by Hello3/0.1@None/None to Hello0/0.1@lasote/stable",
                          self.client.user_io.out)

            json_file = os.path.join(self.client.current_folder, 'tmp.json')
            self.client.run('info . --only=requires --json="{}"'.format(json_file))
            data = json.loads(load(json_file))
            hello0 = data[0]
            self.assertEqual(hello0["reference"], "Hello0/0.1@lasote/stable")
            self.assertListEqual(sorted(hello0["required_by"]),
                                 sorted(["Hello2/0.1@lasote/stable", "Hello1/0.1@lasote/stable", "Hello3/0.1@None/None"]))