Restore the bootstrap script changes to install the SwiftPM-built PackageDescription and PackagePlugin libraries

[abertelrud]

Restores the bootstrap script changes that were partially reverted in #3540, and updates the bootstrap script to filter out the .swiftmodule files on Darwin, leaving just .swiftinterface.

Motivation:

In order to build universal toolchains, the bootstrap script was changed to install the PackageDescription and PackagePlugin libraries built by SwiftPM rather than those built by CMake. When building for more than one architecture, SwiftPM uses XCBuild, but apparently only Xcode versions such as those used on the bot don't properly support the build setting to emit Swift module interface files.

Modifications:

Revert the changes to copy the installed PackageDescription and PackagePlugin from the CMake-built ones, so that they again come from the SwiftPM-built ones. Add filtering-out of .swiftmodule files on Darwin, leaving just .swiftinterface.

Result:

PackageDescription and PackagePlugin will be built universal and will work on Apple Silicon.

[abertelrud]

This needs to wait until @shahmishal gives the go-ahead that the macOS CI bots have been upgraded to something newer than Xcode 12.4.

[abertelrud]

@shahmishal Has this update been completed, e.g. do the builders have Xcode 12.5 or later?

[abertelrud]

@swift-ci please smoke test

[abertelrud]

@shahmishal Has this update been completed, e.g. do the builders have Xcode 12.5 or later?

Actually, I think I got this wrong: they just need 12.4 or later. Previously they had 12.3, which seems to be lacking a necessary bugfix in 12.4. From the logs (based on version numbers of the compiler) it appears that they are using 12.4 now even though ci.swift.org says 12.3. @shahmishal is that correct?

[abertelrud]

I've done some more testing with Xcode 12.3 as well as Xcode 12.4, and I cannot reproduce the original issue of not producing .swiftinterface files with either version. I think one thing that might have contributed to the original confusion is a difference between single-architecture and multi-architecture Swift modules:

• single-architecture Swift modules are built as flat .swiftinterface files
• multi-architecture Swift modules are built as .swiftmodule directories containing .swiftinterface files (along with .swiftmodule and other files

We need to get this merged in order to have universal SwiftPM dylibs in the toolchain. Last time I tried a cross-repository test toolchain build it failed for what seemed to be completely unrelated reasons, but I'll give that another try.

[abertelrud]

The last remaining difference was that the new code to install the library and module didn't filter out the .swiftmodule that was alongside the .swiftinterface. I have updated the PR to do that, so that we are back to exactly what the installed Swift modules from CMake produced. With that, we should be independent of the Xcode version involved, and this can be merged at any time.

[abertelrud]

@swift-ci please smoke test

[abertelrud]

The toolchain test seems to have successfully built SwiftPM, though it failed at a later stage for unrelated reasons. I think this is ready to merge but have also pinged @shahmishal to see whether any other testing is needed. To me this is ready to merge.