[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.

[lorentey]

The changes look good to me! Will this change affect builds of swift-collections that are part of Swift toolchains?

Oh, could you please target this change on the release/1.2 branch rather main? I expect we'll want to quickly ship this.

[Steelskin]

The changes look good to me! Will this change affect builds of swift-collections that are part of Swift toolchains?

Oh, could you please target this change on the release/1.2 branch rather main? I expect we'll want to quickly ship this.

Ah sorry, I opened it as a draft PR because I am waiting for a review on apple/swift-argument-parser#685, which does the same thing but in swift-argument-parser. I intend to update the rest of the libraries used by the toolchain in the same manner and I would like to stay consistent.

[Steelskin]

apple/swift-argument-parser#685 has landed. This is bringing the same changes to this repository. I changed the base branch to release/1.2 as requested. Will this make it back to main automatically or do I need to do something else?

[Steelskin]

I pushed an update that reverts commit 40adeb7, it seems to have been done in error. Other Swift libraries put the .swiftdoc in the .swiftmodule folder.

[lorentey]

Will this make it back to main automatically or do I need to do something else?

You don't need to do anything else -- the changes will get picked up the next time we merge things forward. (This is a manual process, so there can be a delay. But this repo's main should not be used in production.)