This PR resolves warnings:
```
…/.build/checkouts/swift-docc-plugin/Plugins/Swift-DocC Preview/Symbolic Links/SwiftDocCPluginUtilities/BuildGraph/DocumentationBuildGraph.swift:106:17: warning: capture of non-sendable type 'Target.Type' in an isolated closure
104 | // Copy the closure and the target into a block operation object
105 | let new = BlockOperation { [work, task] in
106 | work(task)
| `- warning: capture of non-sendable type 'Target.Type' in an isolated closure
107 | }
108 | operationsByID[task.id] = new
…/.build/checkouts/swift-docc-plugin/Plugins/Swift-DocC Preview/Symbolic Links/SwiftDocCPluginUtilities/BuildGraph/DocumentationBuildGraph.swift:105:41: warning: capture of non-sendable type 'Target.Type' in an isolated closure
103 | }
104 | // Copy the closure and the target into a block operation object
105 | let new = BlockOperation { [work, task] in
| `- warning: capture of non-sendable type 'Target.Type' in an isolated closure
106 | work(task)
107 | }
```
Dmitry Kovba