[51815] introduced the creation of a temporary backup of plugins before updating.
The `move()` (and later, `move_dir()`) call) uses a `$src` parameter.
For Hello Dolly, this is `<path>/wp-contents/plugins/.` (note the period at the end).
For users on Linux and Mac, this doesn't appear to cause any problems.
However, on Windows, the move causes the plugins folder to be moved which then causes a failure when attempting to call `mkdir()`.
This commit skips any plugin whose slug is `'.'` as this slug results in the `$src` value ending in a period.
Follow-up to [51815].
Props costdev, boniu91, hellofromTonya.
Fixes#54543.
Built from https://develop.svn.wordpress.org/trunk@52337
git-svn-id: http://core.svn.wordpress.org/trunk@51929 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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