Fix namespace and imports

Problem/Motivation

drupal-check reports

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  Line   metatag_facebook/src/Plugin/metatag/Group/Facebook.php
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         Class Drupal\facebook\Plugin\metatag\Group\Facebook was not found while trying to analyse it - autoloading is probably not configured properly.
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  Line   metatag_mobile/src/Plugin/metatag/Group/AndroidMobile.php
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         Class Drupal\metatag_mobile\Plugin\metatag\Group\GroupBase not found and could not be autoloaded.
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  Line   metatag_mobile/src/Plugin/metatag/Group/AppleMobile.php
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         Class Drupal\metatag_mobile\Plugin\metatag\Group\GroupBase not found and could not be autoloaded.
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  Line   metatag_mobile/src/Plugin/metatag/Group/Mobile.php
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         Class Drupal\metatag_mobile\Plugin\metatag\Group\GroupBase not found and could not be autoloaded.
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  Line   metatag_mobile/src/Plugin/metatag/Group/WindowsMobile.php
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         Class Drupal\metatag_mobile\Plugin\metatag\Group\GroupBase not found and could not be autoloaded.

Proposed resolution

Fix the namespace and imports