Restructure barrel files & avoid using barrel imports internally

[fubhy]

Just taking note of some things we briefly discussed in PR & related issue comments:

I think it would be good to refactor the imports & exports so that we do not use imports from barrel files (index.ts) internally at all (except maybe in tests).

@jxom I liked your idea to explicitly maintain a set of entrypoint files and entirely avoid barrel files otherwise.

[fubhy]

Goals:

1. Remove the possibility of accidental circular references
2. Simplify (& improve) tree shaking
3. Improve developer experience by not offering multiple different import options when auto-completeing symbol names
4. Improve developer experience when searching for & navigating code references
5. Simplify refactoring (moving & renaming of files) and IDE assistance thereof
6. Manage the library's public interface in the entrypoint files only (already the status quo but more explicit & obvious this way)

Rules:

1. All entrypoints (as defined in package.json exports) are strictly in src/entrypoints/*.ts. There are no other barrel files elsewhere. (h/t @jxom)
2. Entrypoints may not be imported anywhere in src. The only exception would be integration tests, which we don't have a whole lot of atm. and, which I'd argue, would belong in a shared tests dir (possibly outside of src).
3. All exported symbols should be uniquely named within the codebase (currently not the case, likely something we would tackle seperately)