clarify handling of index importing
clarify handling of index importing

Minor code change: make the 'is in node_modules' check resilient
to situations where the entire tsickle source tree is contained
within a node_modules directory.

To make that change, I refactored the function to add a lot more
comments, because I had trouble following the existing logic.

alexeagle/tsickle,alexeagle/tsickle,angular/tsickle,alexeagle/tsickle,angular/tsickle