Enable MemberImportVisibility check on all targets (#44)

Enable MemberImportVisibility check on all targets. Use a standard
string header and footer to bracket the new block for ease of updating
in the future with scripts.

Package.swift

@@ -169,3 +169,14 @@ let package = Package(
     ],
     cxxLanguageStandard: .cxx14
 )
+
+// ---    STANDARD CROSS-REPO SETTINGS DO NOT EDIT   --- //
+for target in package.targets {
+    if target.type != .plugin {
+        var settings = target.swiftSettings ?? []
+        // https://github.com/swiftlang/swift-evolution/blob/main/proposals/0444-member-import-visibility.md
+        settings.append(.enableUpcomingFeature("MemberImportVisibility"))
+        target.swiftSettings = settings
+    }
+}
+// --- END: STANDARD CROSS-REPO SETTINGS DO NOT EDIT --- //