DriverSupport: make some of the private API more public

This extends the visibility of some of the private implementation to the
package level to allow use for testing without `@testable` imports.

Author: compnerd

Sources/DriverSupport/DriverSupportUtils.swift

@@ -56,7 +56,7 @@ public enum DriverSupport {
     // This checks if given flags are supported in the built-in toolchain driver. Currently
     // there's no good way to get the supported flags from it, so run `swiftc -h` directly
     // to get the flags and cache the result.
-    static func checkToolchainDriverFlags(
+    package static func checkToolchainDriverFlags(
         flags: Set<String>,
         toolchain: PackageModel.Toolchain,
         fileSystem: FileSystem