Explicitly import _Concurrency in PackageDescription/BuildSettings.swift (#8850)

In order to reference `MainActor.Type` in certain circumstances
`_Concurrency` must be imported.

Author: plemarquand

Sources/PackageDescription/BuildSettings.swift

@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+import _Concurrency
+
 /// The build configuration, such as debug or release.
 public struct BuildConfiguration: Sendable {
     /// The configuration of the build. Valid values are `debug` and `release`.