doctest: don't open code the module import

Currently, `DoctestModule` does `import_path` on its own. This changes
it to use `importtestmodule` as `Module` does. The behavioral changes
are:

- Much better error messages on import errors.

- Handles a few more error cases (see `importtestmodule`). This
  technically expands the cover of `--doctest-ignore-import-errors` but
  I think it makes sense.

- Considers `pytest_plugins` in the module.

- Populates `self.obj` as properly (without double-imports) as is
  expected from a `PyCollector`.

This is also needed for the next commit.

src/_pytest/doctest.py

@@ -40,7 +40,6 @@ from _pytest.nodes import Item
 from _pytest.outcomes import OutcomeException
 from _pytest.outcomes import skip
 from _pytest.pathlib import fnmatch_ex
-from _pytest.pathlib import import_path
 from _pytest.python import Module
 from _pytest.python_api import approx
 from _pytest.warning_types import PytestWarning
@@ -107,7 +106,7 @@ def pytest_addoption(parser: Parser) -> None:
         "--doctest-ignore-import-errors",
         action="store_true",
         default=False,
-        help="Ignore doctest ImportErrors",
+        help="Ignore doctest collection errors",
         dest="doctest_ignore_import_errors",
     )
     group.addoption(
@@ -561,12 +560,8 @@ class DoctestModule(Module):
                 pass
 
         try:
-            module = import_path(
-                self.path,
-                root=self.config.rootpath,
-                mode=self.config.getoption("importmode"),
-            )
-        except ImportError:
+            module = self.obj
+        except Collector.CollectError:
             if self.config.getvalue("doctest_ignore_import_errors"):
                 skip("unable to import module %r" % self.path)
             else: