tsc --build --watch does not watch wild card directories correctly

[sheetalkamat]

updateWatchingWildcardDirectories is normally used for single project and it mutates map to have new watch directories, Since we have single existingWatchersForWildcards among all projects this is incorrect and not watching wild card directories correctly.
https://github.com/Microsoft/TypeScript/blob/master/src/compiler/tsbuild.ts#L569

[RyanCavanaugh]

@sheetalkamat feel free to bring this into 3.1 milestone if you have time