[cmake] Install libraries in standard directories

[Steelskin]

Previously, libs were installed under lib/swift/${os}. They should be installed in the default library directory for the relevant target system.
In addition, swiftmodules were installed in the older layout format. This changes to use the standard modern layout format for swiftmodules.

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.