Elide SwiftNIO on WASI

Motivation:

SwiftNIO cannot be built for `wasm32-unknown-wasip1`, so SQLKit currently fails
to configure for that platform at all — the failure is in dependency resolution,
before any of the conditional sources in the previous commit get a chance to
matter.

Modifications:

Gate the NIOCore product on `.when(platforms: nonWASIPlatforms)`. Target
dependency conditions are evaluated per platform, so on WASI the product is
simply not linked and the `canImport(NIOCore)` gates select the `async` API.

`.when(platforms:)` can only include, never exclude, so excluding one platform
means enumerating the others; the list is the set SPM 6.1 knows about, noted as
such so it is not extended without also raising the manifest's tools version.

The test target's NIOCore and NIOEmbedded dependencies are gated the same way;
the suite exercises the `EventLoopFuture` API and is not run on WASI.

Result:

On every other platform the resolved dependency set is byte-identical to before.
On WASI the build graph contains no SwiftNIO module — not NIOPosix, not NIOCore,
not NIOConcurrencyHelpers.

Author: scottmarchant

Package.swift

@@ -1,6 +1,11 @@
 // swift-tools-version:6.1
 import PackageDescription
 
+/// This list matches the [supported platforms on the Swift 6.1 release of SPM](https://github.com/swiftlang/swift-package-manager/blob/release/6.1/Sources/PackageDescription/SupportedPlatforms.swift).
+/// Don't add new platforms here unless raising the swift-tools-version of this manifest.
+let allPlatforms: [Platform] = [.macOS, .macCatalyst, .iOS, .tvOS, .watchOS, .visionOS, .driverKit, .linux, .windows, .android, .wasi, .openbsd]
+let nonWASIPlatforms: [Platform] = allPlatforms.filter { $0 != .wasi }
+
 let package = Package(
     name: "sql-kit",
     platforms: [
@@ -24,7 +29,11 @@ let package = Package(
             dependencies: [
                 .product(name: "Collections", package: "swift-collections"),
                 .product(name: "Logging", package: "swift-log"),
-                .product(name: "NIOCore", package: "swift-nio"),
+                // SwiftNIO does not support wasm32-unknown-wasip1. Target dependency conditions
+                // are evaluated per platform, so on WASI NIOCore is simply not linked and the
+                // `EventLoopFuture` surface drops out via `#if canImport(NIOCore)`; the async
+                // surface is unaffected.
+                .product(name: "NIOCore", package: "swift-nio", condition: .when(platforms: nonWASIPlatforms)),
             ],
             swiftSettings: swiftSettings
         ),
@@ -38,8 +47,9 @@ let package = Package(
         .testTarget(
             name: "SQLKitTests",
             dependencies: [
-                .product(name: "NIOCore", package: "swift-nio"),
-                .product(name: "NIOEmbedded", package: "swift-nio"),
+                // The test suite exercises the SwiftNIO surface, so it is not built for WASI.
+                .product(name: "NIOCore", package: "swift-nio", condition: .when(platforms: nonWASIPlatforms)),
+                .product(name: "NIOEmbedded", package: "swift-nio", condition: .when(platforms: nonWASIPlatforms)),
                 .target(name: "SQLKit"),
                 .target(name: "SQLKitBenchmark"),
             ],