[TSCBasic] Make init public (#476)

This is mismatching the other init which is public.

Sources/TSCBasic/Lock.swift

@@ -77,7 +77,7 @@ public final class FileLock {
     /// Create an instance of FileLock at the path specified
     ///
     /// Note: The parent directory path should be a valid directory.
-    internal init(at lockFile: AbsolutePath) {
+    public init(at lockFile: AbsolutePath) {
         self.lockFile = lockFile
     }