Fix imports for non-barrel files ending with index

[naeramarth7]

The previous replacer removed the trailing index of every import path, no matter if it is a barrel file or just ends in index.

Examples:

# Import from ./_index.ts resolves to:
import { module } from './_';

# import from ./some/path/the-index resolves.ts to:
import { module } from './some/path/the-';

This fix replaces only index if it is a barrel file, so the above examples will be imported correctly while barrel file imports drop the final index from the path.

This resolves buehler/typescript-hero#163.

[coveralls]

Coverage increased (+0.09%) to 82.741% when pulling 5eed11d on naeramarth7:bugfix/imports-ending-with-index into 32470ff on TypeScript-Heroes:develop.

[coveralls]

Coverage increased (+0.09%) to 82.741% when pulling 6c5a47a on naeramarth7:bugfix/imports-ending-with-index into 32470ff on TypeScript-Heroes:develop.

[buehler]

Thank you!