Rename _CollectionsUtilities to InternalCollectionsUtilities

[lorentey]

We are forced to do this due to an Xcode issue (#364) with underscored module names.

This module remains unstable, despite the removal of the underscore. Directly importing it from outside the package remains unsupported. This module and/or its entire contents may go away or change in incompatible ways in any future swift-collections release.

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.

[lorentey]

@swift-ci test

[Azoy]

Never change Xcode ❤️