Put URL downloading back into Data(contentsOf:)

[parkera]

The new Data reading API doesn't support non-file URLs on non-Darwin systems. This could be a compatibility issue.

see also: https://forums.swift.org/t/data-contentsof-and-new-swift-foundation/73632

[jmschonfeld]

#820 and swiftlang/swift-corelibs-foundation#5049 will resolve this by adding an up-call to FoundationNetworking on main, I'll also cherry-pick this back to release/6.0 for Swift 6

[jmschonfeld]

Fixed on main, #826 and swiftlang/swift-corelibs-foundation#5058 are the 6.0 cherry-pick PRs

[jmschonfeld]

6.0 PRs are now merged as well - marking this as resolved