Cannot redeclare GuzzleHttp\Psr7\_caseless_remove()

[samjameswatson]

Bug Description

Attempting to activate SiteKit on a WordPress install that already has an active plugin that loads GuzzleHttp causes a fatal error as it can't be redeclared.

Steps to reproduce

Install and Activate any plugin that loads GuzzleHttp (in my case it was /~https://github.com/humanmade/S3-Uploads but I believe this would affect any plugin making use of the AWS SDK).

Then attempt to activate SiteKit.

Additional Context

• PHP Version: 7.3.2
• Plugin Version: 1.0.0-beta.1.0.2

I came across this other GitHub issue that seems related - aws/aws-sdk-php#1616.
They suggest to load functions_include.php instead of functions.php to resolve the issue. Not sure how relevant that is here but hope it helps.

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Acceptance criteria

Implementation Brief

Changelog entry

[swissspidy]

Related: #223

[swissspidy]

Closing as a duplicate of #223 where we're already tracking this. See also #246, #41.