In order to support backdeployment of Swift concurrency features to older operating systems, Swift builds the concurrency APIs into a separate library which can be shipped separately from the Swift standard library. This swift_Concurrency library is exposed as an implicit module called _Concurrency, which is re-exported from the Swift standard library.
However, the Swift concurrency classes and types still belong to this implementation-only library, so in order to disambiguate, you'll need to write _Concurrency.Task instead of Swift.Task.
You can also see this in Xcode: if you refer to Task in a new project, and ^-⌘-click on the symbol to jump to its definition, you'll find it in the _Concurrency library interface:
Task interface definition in Xcode
Instead of explicitly writing out _Concurrency.Task in order to disambiguate everywhere, consider the following:
You can create a typealias for the type (e.g. typealias SwiftTask = _Concurrency.Task) if you prefer your own Task type taking precedence, and don't want to rename it
Alternatively, depending on whether it makes sense for your project (and is reasonable in context): this might be an opportunity to consider a more specific name for your type to possibly avoid both nominal and logical conflicts. async/await and Task are major new language/stdlib features that are going to be influential for a long time, so it may make sense to pick a less general word if the stdlib is going to be reserving that one going forward