Fix build failures for Swift 5

[kphrx]

fix #870

This fixes build failures caused by:

• the inability to use conditional attributes in 5.7
  • refs Fix ArgumentParser build failure for WASI (WebAssembly System Interface) #794
  • If merge Support async custom completion functions for AsyncParsableCommand via async/await #855 first, unnecessary cec9c2f, so needs drop this.
• the inability to use access level modifiers with imports in Swift 5 (they are available in 5.9 and later with the experimental feature flag)
  • refs Use Foundation Essentials when possible #804
• macro target type not exists in Swift 5.8
  • refs Adopt upcoming Swift language feature MemberImportVisibility #803

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rgoldberg]

My PR #855 removes the usage of Dispatch.DispatchSemaphore, so many of the changes in this PR would be unnecessary if my PR is merged.

Given that my PR improves functionality & that many changes in this PR conflict with my PR, I think it makes sense to try to merge mine first, then rebase this PR so it fixes only the remaining Swift 5 build issues.

[natecook1000]

@rgoldberg I'm planning to land this PR first, so that we can have one more patch release with the current supported list of Swift versions. In the next minor release I'd like to raise the minimum to Swift 6 to cut down on the level of testing/support.

[rgoldberg]

@natecook1000 Would't bumping the minimum Swift version require a new major version for semantic versioning? Does SAP not use semantic versioning?

[natecook1000]

Swift Package Manager resolves packages with an understanding of which Swift versions are supported, so for Swift versions prior to 6.0, they'll just see 1.7.1 as the last available version.