Enable MemberImportVisibility upcoming feature in Foundation

[parkera]

Enable the MemberImportVisibility compiler flag, which ensures that we don't accidentally depend on implicit imports in our sources.

[parkera]

@swift-ci test

[marcprux]

I think this may have broken my hourly Android build (for 6.1 and 6.2-nightly, but not 6.0): https://github.com/swift-everywhere/swift-package-builds/actions/runs/14801693208/job/41561703634#step:32:2259. Does this look like it could be plausibly related to the StrictConcurrency change?

/home/runner/work/swift-package-builds/swift-package-builds/swift-foundation/Sources/FoundationEssentials/Error/CocoaError+FilePath.swift:76:37: error: initializer 'init(rawValue:)' is not available due to missing import of defining module 'Android'
 18 | #elseif canImport(Bionic)
 19 | @preconcurrency import Bionic
 20 | #elseif canImport(Glibc)
    | `- note: add import of module 'Android'
 21 | @preconcurrency import Glibc
 22 | #elseif canImport(Musl)
    :
 74 |         // (130280235) POSIXError.Code does not have a case for EOPNOTSUPP
 75 |         guard errno != EOPNOTSUPP else { return nil }
 76 |         guard let code = POSIXError.Code(rawValue: errno) else {
    |                                     `- error: initializer 'init(rawValue:)' is not available due to missing import of defining module 'Android'
 77 |             fatalError("Invalid posix errno \(errno)")
 78 |         }

Replacing #elseif canImport(Bionic) @preconcurrency import Bionic with #elseif canImport(Android) @preconcurrency import Android does fix the issue, but I'm curious what might have triggered that.

[marcprux]

cc: @finagolfin