Various: Ensure WP CLI is defined before extending

[kraftbj]

After #23039 we are attempting to load all autoloaded files upon upgrade to we don't have mid-deploy fatals like we've had in the past. But, this is creating an issue since the Search package's CLI class is now being loaded without WP CLI being present.

Changes proposed in this Pull Request:

• Ensures WP CLI is present before extending the class. This is helpful in situations where a class is autoloaded without a gate.

Jetpack product discussion

p1646255328527279-slack-CBG1CP4EN

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Not real sure. Install this on a test site, then have an upgrade routiene to a newer version than what is produced by this patch I suppose.

[github-actions]

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Beta plugin:

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Jetpack plugin:

• Next scheduled release: April 5, 2022.
• Scheduled code freeze: March 29, 2022.