EntryFilesAnalyser: add option to ignore entry file that doesn't exist

[fraxken]

Current API eg analyse throw if provided with entry files that don't exist, the idea would be to provide a new option to the constructor to ignore the entry if they don't exist.

const entries = ["index.js"]; // this file doesn't exist
const efa = new EntryFilesAnalyser({
  ignoreENOENT: true
});

for await (const fileReport of efa.analyse(entries)) {
}

// No error throw

I guess we can use DiGraph and dependencies property to tell the number of provided entries doesn't match with the number of vertices.