PR: Update layout setup to discard areas where the base plugin doesn't exist

[dalthviz]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Following #15177 if a dockable plugin is pruned and it's used to define an area for a layout (for example the Outline Explorer) the related area should not be taken into account when applying the layout. Prevents seeing errors like the following when a plugin is pruned:

Traceback (most recent call last):
  File "C:\Users\Daniel\Documents\Spyder\spyder\spyder\plugins\layout\container.py", line 125, in <lambda>
    triggered=lambda: self.quick_layout_switch('default'),
  File "C:\Users\Daniel\Documents\Spyder\spyder\spyder\plugins\layout\container.py", line 393, in quick_layout_switch
    possible_current_layout = self._plugin.quick_layout_switch(index)
  File "C:\Users\Daniel\Documents\Spyder\spyder\spyder\plugins\layout\plugin.py", line 320, in quick_layout_switch
    self.setup_default_layouts(index, settings)
  File "C:\Users\Daniel\Documents\Spyder\spyder\spyder\plugins\layout\plugin.py", line 273, in setup_default_layouts
    layout.set_main_window_layout(self.main, self.get_dockable_plugins())
  File "C:\Users\Daniel\Documents\Spyder\spyder\spyder\plugins\layout\api.py", line 375, in set_main_window_layout
    plugin = main_window.get_plugin(plugin_id)
  File "C:\Users\Daniel\Documents\Spyder\spyder\spyder\app\mainwindow.py", line 170, in get_plugin
    'Plugin "{}" not found!'.format(plugin_name))
spyder.api.exceptions.SpyderAPIError: Plugin "outline_explorer" not found!

Issue(s) Resolved

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: dalthviz

[ccordoba12]

Thanks @dalthviz! Looks good to me.