fix (sucrase): resolve directory imports

[larrybotha]

Rollup Plugin Name: sucrase

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

• Plugin sucrase is not resolving directories #239

Description

Attempting to import index.ts via its directory fails:

// my-dir/index.ts
export default 'hello';

// some-other-file.ts
import hello from './my-dir'; // <= fails

This PR fixes this issue, while adhering to Node's strategy of first attempting to resolve the filename for a file named my-dir.ts, and then attempting to resolve my-dir/index.ts: https://www.typescriptlang.org/docs/handbook/module-resolution.html#how-nodejs-resolves-modules

[shellscape]

LGTM. Thank you for opening this PR. I'll request review from a few others. When we have two approvals, we'll merge and put it on the release schedule.

[shellscape]