Make deriveTargetSwiftSDK public (swiftlang#7925)

Used by swiftlang/sourcekit-lsp#1643

### Motivation:

Allows us to achieve parity between sourcekit-lsp's configuration
options and SwiftPM's `--triple`/`--swift-sdk`.

### Modifications:

Make `SwiftSDK.deriveTargetSwiftSDK` public

### Result:

See swiftlang/sourcekit-lsp#1643

Sources/PackageModel/SwiftSDKs/SwiftSDK.swift

@@ -656,7 +656,6 @@ public struct SwiftSDK: Equatable {
     }
 
     /// Computes the target Swift SDK for the given options.
-    @_spi(SwiftPMInternal)
     public static func deriveTargetSwiftSDK(
       hostSwiftSDK: SwiftSDK,
       hostTriple: Triple,