Xcode 13 produces error Cannot find type 'SnippetBuilder' in scope

[sofiaromorales]

When trying to generate documentation from the main branch with Swift 5.6 an error message appears Cannot find type 'SnippetBuilder' in scope, this does not happen in 3e3305b

Checklist

• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue.

Expected behavior

Documentation to be generated when using Swift 5.6

Actual behavior

Error Cannot find type 'SnippetBuilder' in scope is thrown

Steps to Reproduce

Using Swift 5.6 and running the command swift package generate-documentation against the main branch produces the error

Swift-DocC Plugin Version Information

Swift-DocC plugin version: 5f2e247 (current HEAD -> main)
Swift Compiler version: swift-driver version: 1.45.2 Apple Swift version 5.6 (swiftlang-5.6.0.323.62 clang-1316.0.20.8)