v1.7.3.1: Better handling of projects with a node_modules directory

Some npm packages, such as angular2, include both .d.ts files and source
.ts files. The source files should not be considered part of the
enclosing project; they will cause duplicate definition errors and slow
down error checking.

* Support tsconfig.json's "files" and "exclude" sections. Make
  host.getScriptFileNames() return only those files specified therein.
  Other files may still be brought in via imports or <reference>s.
* If there are multiple tsconfig.json files, use the one with the
  shortest path (instead of the one most recently indexed).