Autoloader: Implementing composer autoloader with classmap

[wjrosa]

Fixes STRIPE-507
See peNR48-1sM-p2#comment-879

Changes proposed in this Pull Request:

As part of the autoloader implementation, this PR changes the existing version to use Composer's classmap version instead. This should provide Composer with useful features such as the auto-generation of the classmap file and cache.

Testing instructions

Install

• Checkout to this branch on your local environment (dev/implementing-psr-4-autoloader-without-namespaces)
• Run npm run build:deps
• Confirm you get no fatal errors related to missing files upon accessing the admin interface or the checkout page as a shopper

Build

• Run npm run build
• Confirm the extension .zip file is correctly generated, and it now includes the vendor folder

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Changelog entry

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Changelog Entry Comment

Comment

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[github-actions]

📈 PHP Unit Code Coverage Report

Package	Line Rate	Health
includes/WC_Stripe_Customer.php	40%	❌
Summary	44% (7252 / 16355)	❌

[daledupreez]

@wjrosa, can you change the target of this PR to the try/add-basic-classmap-autoloader branch? The current target branch is dev/renaming-the-main-extension-files, which includes PSR-4 class renames, which aren't necessary for the classmap to function.

[wjrosa]

@daledupreez I messed it up with the rebase, so I created another one: #4406