Extensions: Fix webpack config

[ockham]

Turns out _.merge() caused CopyWebpackPlugin to override the first item in webpackConfig.plugins (DefinePlugin) 🙄

Discovered while working on Automattic/wp-calypso#32083.

Changes proposed in this Pull Request:

Extensions: Fix webpack config by adding CopyWebpackPlugin through destructuring rather than _.merge().

Testing instructions:

yarn distclean && yarn ci
yarn build-extensions

Verify that the above is true (e.g. by outputting the result of that _.merge() call on the master branch).

Proposed changelog entry for your changes:

• N/A

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello ockham! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D26610-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: April 29, 2019.
Scheduled code freeze: April 22, 2019

Generated by 🚫 dangerJS against fd165a7

[simison]

Nice catch; those definitions are used in so few places in blocks it's hard to catch without console logging what's going on.

Confirmed that CopyPlugin appears at the bottom of the list and original webpackConfig.plugins is intact.

Let's make sure to document this in docs (Automattic/wp-calypso#31679). Example from Gutenberg; https://github.com/WordPress/gutenberg/pull/14590/files