Fix building for Xcode 13 and Swift 5.5

So the issue is that in Xcode 13 there are new contents where before just were two directories to the different libPackageDescription like so:
shibapm · Sep 28, 2021 · e9f59db · e9f59db
1 parent 6e7415d
commit e9f59db
Showing 1 changed file with 3 additions and 1 deletion.
diff --git a/Sources/PackageConfig/Package.swift b/Sources/PackageConfig/Package.swift
@@ -114,7 +114,9 @@ enum Package {
         let fileManager = FileManager.default
 
         let swiftPMDir = swiftPath.replacingOccurrences(of: "bin/swiftc", with: "lib/swift/pm")
-        let versions = try! fileManager.contentsOfDirectory(atPath: swiftPMDir).filter { $0 != "llbuild" }
+        let versions = try! fileManager.contentsOfDirectory(atPath: swiftPMDir)
+            .filter { $0 != "llbuild" }
+            .filter { $0.first?.isNumber ?? false }
 
         let latestVersion = versions.sorted().last!
         var spmVersionDir = latestVersion