Consider importing mu-plugins directories

[danielbachhuber]

We currently ignore mu-plugins directories in our import workflow. We should consider properly importing them.

[wojtekn]

From https://developer.wordpress.org/advanced-administration/plugins/mu-plugins/:

[...] For web hosts, mu-plugins are commonly used to add support for host-specific features, especially those where their absence could break the site.

It makes sense to treat those as host-specific features. Studio puts different polyfills there, and any host or local app could do the same.

@danielbachhuber how this addition would help you?

[danielbachhuber]

@wojtekn I'm not sure I agree with that description. I think mu-plugins can often be use for site-specific code.

[wojtekn]

@danielbachhuber Now Playground supports loading polyfill my-plugins from Playground-specific directory (/internal/shared/mu-plugins), so they won't clutter WordPress WP Admin: https://github.com/WordPress/wordpress-playground/blob/b81c81d9636989acfcbb7102e618ee840072b958/packages/playground/wordpress/src/index.ts#L17

We could move our polyfills there and support importing and exporting the default mu-plugins/ directory. Users who don't want them can always delete them manually after the import.

[danielbachhuber]

@wojtekn Nice!

[fredrikekelund]

Related to #585 because both issues cover which files are copied from the import archive.

Fixing #585 will likely mean importing ALL content from the archive, which would effectively also fix this issue, but it might make sense to add some special checks or validation for mu-plugins.