normalizeSlashes should probably no-op on *nix

As @dmichon-msft pointed out, Linux file names can contain backslashes, so indiscriminately converting them into forward slashes is probably a bad idea. Redefining normalizeSlashes as the identity function on non-Windows platforms would also be a perf win, since it's very hot code. The biggest concern is that someone might be depending on this (e.g. I think you can use backslashes in paths in tsconfig.json).