build: repair the ability to build with build tree artifacts

[compnerd]

This restores the ability to build swift-collections with build tree
artifacts for the toolchain. The newly introduced dependency on
Foundation in PriorityQueueModule requires linking against Foundation
and dispatch in the case that the system does not provide the libraries
and you do not have a complete toolchain on hand.

Replace this paragraph with a description of your changes and rationale. Provide links to an existing issue or external references/discussions, if appropriate.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[compnerd]

@swift-ci please test

[lorentey]

The newly introduced dependency on
Foundation in PriorityQueueModule

This was a mistake, presumably triggered by Xcode's insistence on adding an import Foundation statement to new Swift files. The correct fix would be to remove the dependency!

[compnerd]

Sure, removal is fine in my book as well. Let me see if I can whip up a patch for that.

[compnerd]

Trying locally and on CI with #118