Have --package usage respect mypy_path (#6926)

Fixes #6595

Author: abatilo

mypy/main.py

@@ -755,7 +755,7 @@ def add_invertible_flag(flag: str,
     # Set target.
     if special_opts.modules + special_opts.packages:
         options.build_type = BuildType.MODULE
-        search_paths = SearchPaths((os.getcwd(),), tuple(mypy_path()), (), ())
+        search_paths = SearchPaths((os.getcwd(),), tuple(mypy_path() + options.mypy_path), (), ())
         targets = []
         # TODO: use the same cache that the BuildManager will
         cache = FindModuleCache(search_paths, fscache)

mypy/test/testpep561.py

@@ -216,6 +216,40 @@ def test_typedpkg(self) -> None:
                 venv_dir=venv_dir,
             )
 
+    def test_mypy_path_is_respected(self) -> None:
+        packages = 'packages'
+        pkg_name = 'a'
+        with tempfile.TemporaryDirectory() as temp_dir:
+            old_dir = os.getcwd()
+            os.chdir(temp_dir)
+            try:
+                # Create the pkg for files to go into
+                full_pkg_name = os.path.join(temp_dir, packages, pkg_name)
+                os.makedirs(full_pkg_name)
+
+                # Create the empty __init__ file to declare a package
+                pkg_init_name = os.path.join(temp_dir, packages, pkg_name, '__init__.py')
+                open(pkg_init_name, 'w', encoding='utf8').close()
+
+                mypy_config_path = os.path.join(temp_dir, 'mypy.ini')
+                with open(mypy_config_path, 'w') as mypy_file:
+                    mypy_file.write('[mypy]\n')
+                    mypy_file.write('mypy_path = ./{}\n'.format(packages))
+
+                with self.virtualenv() as venv:
+                    venv_dir, python_executable = venv
+
+                    cmd_line_args = []
+                    if python_executable != sys.executable:
+                        cmd_line_args.append('--python-executable={}'.format(python_executable))
+                    cmd_line_args.extend(['--config-file', mypy_config_path,
+                                          '--package', pkg_name])
+
+                    out, err, returncode = mypy.api.run(cmd_line_args)
+                    assert returncode == 0
+            finally:
+                os.chdir(old_dir)
+
     def test_stub_and_typed_pkg(self) -> None:
         self.simple_prog.create()
         with self.virtualenv() as venv: