fix external module detection on Windows

[a-type]

Fixes #87

Absolute path detection was not working for Windows users. I updated the util to use Node's built-in function.

I also created a regression test in the testing suite to ensure that exports defined in other files are correctly included in the final library build.

[a-type]

Another problem that's related: the default tsconfig.json adds src/* to the paths, but doing so means that TS will resolve auto-imported modules relative to src (so if I auto-imported something from src/utils.ts, it would come in as import { foo } from 'utils'. This causes these modules to also be marked as external. That's likely to confuse people. Either the default tsconfig shouldn't use the absolute resolution in src, or the external check will need to be made a bit smarter. I consider that outside the scope of this particular issue (Windows support out of the box).

[jaredpalmer]

Nice.

[agilgur5]

@allcontributors please add @a-type for code, test, bug

[allcontributors]

@agilgur5

I've put up a pull request to add @a-type! 🎉