Fix JS/TS collisions

[jeffdaley]

Fixes JS/TS collisions that were causing this warning:

WARNING: Detected collisions between .js and .ts files of the same name. This can result in nondeterministic build output; see https://git.io/JvIwo for more information.

Using guidance from the issue thread, we now underscore the filenames of internal TS files (e.g., session.ts → _session.ts) and then add a non-underscored .js file (e.g., session.js) that specifies the internal export.

• Import path updates