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.

Fix pytest-dev#11929.

Figured out what's going on. We have the following collection tree:

```
<Dir pyspacewar>
  <Dir src>
    <Package pyspacewar>
      <Package tests>
        <DoctestModule test_main.py>
          <DoctestItem pyspacewar.tests.test_main.doctest_main>
```

And the `test_main.py` contains an autouse fixture (`fake_game_ui`) that
`doctest_main` needs in order to run properly. The fixture doesn't run!
It doesn't run because nothing collects the fixtures from (calls
`parsefactories()` on) the `test_main.py` `DoctestModule`.

How come it only started happening with commit
ab63ebb? Turns out it mostly only
worked accidentally. Each `DoctestModule` is also collected as a normal
`Module`, with the `Module` collected after the `DoctestModule`. For
example, if we add a non-doctest test to `test_main.py`, the collection
tree looks like this:

```
<Dir pyspacewar>
  <Dir src>
    <Package pyspacewar>
      <Package tests>
        <DoctestModule test_main.py>
          <DoctestItem pyspacewar.tests.test_main.doctest_main>
        <Module test_main.py>
          <Function test_it>
```

Now, `Module` *does* collect fixtures. When autouse fixtures are
collected, they are added to the `_nodeid_autousenames` dict.

Before ab63ebb, `DoctestItem` consults
`_nodeid_autousenames` at *setup* time. At this point, the `Module` has
collected and so it ended up picking the autouse fixture (this relies on
another "accident", that the `DoctestModule` and `Module` have the same
node ID).

After ab63ebb, `DoctestItem` consults
`_nodeid_autousenames` at *collection* time (= when it's created). At
this point, the `Module` hasn't collected yet, so the autouse fixture is
not picked out.

The fix is simple -- have `DoctestModule.collect()` call
`parsefactories`. From some testing I've done it shouldn't have negative
consequences (I hope).