Improve pathExists and isDirectory performance once graceful-fs supports it

[markjm]

Once this change is merged: isaacs/node-graceful-fs#221, stop throwing errors in below places:

license-webpack-plugin/src/WebpackFileSystem.ts

Line 21 in e70c57b

this.fs.statSync(filename);

license-webpack-plugin/src/WebpackFileSystem.ts

Line 47 in e70c57b

isDir = this.fs.statSync(dir).isDirectory();

Since we expect this can be null, we skip a lot of overhead in creating these errors. Skipping these errors is over a 3x speedup for these calls
nodejs/node#33716

[xz64]

Implemented by @markjm in #114