Move all submodules to external directory

[MrJul]

What does the pull request do?

This PR moves all submodules to a common external folder.
This is done to avoid any unexpected issue occurring by having submodules in the src folder.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055514-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055522-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055524-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Trouble may arise when pulling changes after this PR.

First, make sure the submodules are updated:
git submodule update --init --recursive

Git may not correctly remove some old submodule folders ("unable to rmdir 'src/Avalonia.Controls.DataGrid': Directory not empty")

If the warning above appears, or if you've got duplicated symbol errors when building, clean your working directory and all submodules:
git clean -ffxd