DocCPlugin docs failing

[finestructure]

I noticed this after the switchover from apple to swiftlang - there's no documentation link on the package page anymore, because the doc builds are actually failing (both on main and for 1.3.0):

Working copy of https://github.com/apple/swift-docc-plugin resolved at 1.3.0
error: unable to override package 'SwiftDocCPlugin' because its identity 'swift-docc-plugin' doesn't match override's identity (directory name) 'spi-builder-workspace'
error: fatalError
error: unable to override package 'SwiftDocCPlugin' because its identity 'swift-docc-plugin' doesn't match override's identity (directory name) 'spi-builder-workspace'
error: multiple similar targets 'Snippets', 'Swift-DocC', 'Swift-DocC Preview' and 1 others appear in package 'spi-builder-workspace' and 'swift-docc-plugin', this may indicate that the two packages are the same and can be de-duplicated by using mirrors. if they are not duplicate consider using the `moduleAliases` parameter in manifest to provide unique names

We're checking out into spi-build-workspace and that seems to be causing problems when generating the docs?

cc @franklinsch

[finestructure]

Revert disabled doc test once fixed: https://github.com/SwiftPackageIndex/SwiftPackageIndex-Server/pull/3203

[finestructure]

This happens, because we exempt apple/swift-docc-plugin from our automatic plugin mechanism. We now need to exempt swiftlang/swift-docc-plugin.

[finestructure]

Fixed in https://gitlab.com/finestructure/swiftpackageindex-builder/-/merge_requests/298 (not yet deployed)

[finestructure]

Deployed and docs regenerated.

PR to re-enable doc test: https://github.com/SwiftPackageIndex/SwiftPackageIndex-Server/pull/3222