refactor: Scoped imports

[CrownedPhoenix]

Audits all imports of Dispatch and Foundation to use scoped imports instead of global imports.

I'm doing this as a very non-disruptive bit of lookahead for a hope of getting someday getting GRDB to build for WebAssembly. Been down this road before and Foundation and Dispatch are libraries that have some bits that don't compile well for WASM. My intent with this changeset is to make it easier down the road to identify the symbols and places that need to be #if-def'd around and/or implemented in a WASM-friendly way.

TL;DR This will make dependency pruning down the road much simpler.

Pull Request Checklist

• CONTRIBUTING: You have read https://github.com/groue/GRDB.swift/blob/master/CONTRIBUTING.md
• BRANCH: This pull request is submitted against the development branch.
• DOCUMENTATION: Inline documentation has been updated. (N/A)
• DOCUMENTATION: README.md or another dedicated guide has been updated. (N/A)
• TESTS: Changes are tested. (N/A)
• TESTS: The make smokeTest terminal command runs without failure. (N/A)

[groue]

Hello @CrownedPhoenix,

Thank you for this pull request!

Does GRDB build for WebAssembly with those changes?

[CrownedPhoenix]

Does GRDB build for WebAssembly with those changes?

No - or at least I don't think it will (I haven't tried yet because I am pretty sure DispatchQueue won't compile for WASM). This is merely preparatory so that in the future when we (myself or one of my colleagues) come back around to try to get WASM building it will be easier to do the dependency analysis/pruning.