Plugin Directory: Disallow changing the slug to an existing plugin's slug

[SergeyBiryukov]

Background: ​discussion in #meta with @otto42 and @ipstenu.

Under some circumstances, it's possible to have plugins with duplicate slugs:

1. Open a plugin with a new (Pending Initial Review) status for editing.
2. Change the slug to another existing plugin's slug.
3. Click "Mark as Pending".
4. There are now two plugins with the same slug. Whichever one is updated next will get a -2 appended to its slug, causing some confusion.

Changing the slug to another existing plugin's slug should be disallowed.

Some technical details: this happens because ​wp_unique_post_slug() bails early when dealing with a pending post. Both ​get_sample_permalink() and ​wp_ajax_inline_save() have a hack to prevent that by faking a'publish' status, but get_sample_permalink() does not run on Edit Plugin screen, so some additional checks are needed.

[SergeyBiryukov]

3011.patch​ shows an error when trying to use an existing plugin's slug, either on Edit Plugin screen or via Quick Edit.

[Otto42]

In 5727:

Plugin Directory: Prevent plugin slugs from clashing. Fixes #3011. props @SergeyBiryukov