[CMake] Reduce path lengths in single-module build

[jmschonfeld]

Previously, the single module build would add each modules sources to a different list, and then we would combine the list prepending each element with a path like ../BitArray or ../RopeModule. This resulted in extremely long build paths when built as a CMake FetchContent dependency which exceeded the maximum path length on Windows. To resolve this issue, we now create the single module target at the Sources level, and then call target_sources in each subdirectory with a normal relative path such as BitArray+Description.swift. This results in significantly smaller build directory structures keeping the path length within the Windows maximum and also cleans up the CMake a bit.

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]

Huh! 👍