Ignore special folder contents

[Kuniwak]

Ignore missing metas for contents of special folders for 2023.3.0b4 and 2022.3.x, 2021.3.x:

From Unity 2023.1.0a24 onwards a behaviour change to how Unity handles loadable plugin directories has been introduced.

Previously, when importing directories of types .androidlib, .bundle, .framework, and .plugin, .meta files would be created for each asset or directory within them. This caused lots of headaches as it resulted in the introduction of unexpected files into those folders, and multiple workarounds have been implemented in order to remove them for final builds and store submissions.

To address this issue, we have modified the Editor code such that the creation of .meta files in loadable plugin directories is prevented.
Additionally, if those directories already have .meta files in them, the .meta files are ignored when making a game build. We decided on that instead of deleting the files in an upgrade step as that would cause unnecessary version control noise when opening the Editor.

https://forum.unity.com/threads/loadable-plugin-directory-import-behaviour-change-androidlib-bundle-framework-and-plugin.1381113/

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[keigoi]

LGTM