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.