Plugin Directory: Update upload prevention checks to include display name

[Ipstenu]

We already block people from using slugs (permalinks) of existing plugins on .org and we block them if a permalink is in use by 'enough' people outside of .org.

However, since we altered the submissions to build the permalink off the display name, and a number of plugins (properly) have a different display name than permalink, we end up with people trying to upload plugins that exist and it gets by the checks.

For a real world example: Akismet.

The permalink is 'akismet' and the display name is "Akismet Spam Protection" so when someone tries to upload it, they get a permalink of 'akismet-spam-protection' and we reject it.

It would save time, and reduce human error, if we also checked for display name usage, in and OUT of .org, on upload.

Side benny? Forcible encouragement of better names.

[dd32]

In 12165:

Plugin Directory: Upload checks: Prevent an upload of a plugin with a matching title/name (both in the plugin.php and readme.txt files).

Fixes #5880.