Add jetpack-composer-plugin to wpcomsh and mu-wpcom-plugin

[anomiex]

Proposed changes:

The wp i18n command ignores files under vendor/, on the assumption that Composer packages probably aren't using WordPress's i18n functions. This doesn't hold true for many of our packages, such as jetpack-mu-wpcom.

Our workaround for that is a Composer plugin that installs "jetpack-library" packages under jetpack_vendor/ instead, which wp i18n does not ignore.

See Problem 5 at
https://developer.wordpress.com/2022/01/06/wordpress-plugin-i18n-webpack-and-composer/ for details.

Presumably we don't need to worry about Problem 6 for these plugins, as they're only used on WordPress.com and we'd presumably have heard about it already if that problem wasn't worked around somehow at the platform level there.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1737162933817909-slack-C05Q5HSS013
Related to p58i-jdp-p2 and 170251-ghe-Automattic/wpcom

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

No

Testing instructions:

• CI happy?
• Check build artifacts. Files should be moved from vendor/ to jetpack_vendor/.
• Stuff should work the same as it did before.

[github-actions]

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Mu Wpcom plugin:

• Next scheduled release: WordPress.com Simple releases happen semi-continuously (PCYsg-Jjm-p2).

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

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Wpcomsh plugin:

• Next scheduled release: Atomic deploys happen twice daily on weekdays (p9o2xV-2EN-p2).

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[anomiex]

Looks like deploying this one to Simple may be a pain. I'll probably want to add in the following:

• In the update diff, create symlinks from vendor/automattic/* to jetpack_vendor/automattic/*. And vice versa in the active directory.
• In the switch diff, also update a bunch of require_onces to look in jetpack_vendor/. The symlinks should prevent any mid-upgrade fatals.

[tbradsha]

Looks like a reasonable iteration toward the goal. I'm not sure I understand the failed WP.com tests, though...e.g. why would linting be complaining?

[anomiex]

All the wpcom test failures are because of files like wp-content/mu-plugins/masterbar.php that are doing like

require_once JETPACK_MU_WPCOM_PLUGIN_LOADER_PATH . '/vendor/automattic/jetpack-masterbar/src/wp-posts-list/bootstrap.php';

which will need to be changed to /jetpack_vendor/ (i.e. the second bullet in my comment above).