Enhancements to the release build script

[cmcgee1024]

Add macOS support for making releases there.

Rewrite the release builder script in Swift for better reproducibility and ergonomics. It runs using this command:

swift run build-swiftly-release

Perform necessary system checks up front so that anyone running the script can make any necessary adjustments so that the release can be built.

Print the location of the final release product file at the end for easy of finding the path to copy to the final release destination.

[cmcgee1024]

@swift-ci test macOS

[cmcgee1024]

Addresses a portion of #146

[cmcgee1024]

TODO: include all of the relevant licenses into the release artifact, including libarchive.

[cmcgee1024]

@swift-ci test macOS

[shahmishal]

We should update the logic to allow inputs for identifier and sign cert for macOS .pkg