Closes #7

• We use the new Griffe on_package_loaded event to statically analyse things only once the whole package was loaded, to avoid the lookup errors described in Could not resolve alias when running griffe + this on Strawberry's codebase #7.
• Code is made more consistent between the static and dynamic helpers (though the dynamic ones still need to be implemented - this can be done later).
• Unit tests added ☔ (only for uses described in PEP 727, not the "enhanced" features yet)

@patrick91: I tested on strawberry, and the code duplication workaround isn't needed anymore.
@tiangolo : I checked FastAPI's docs with these changes, all seems good.

Don't hesitate to check for yourselves of course, this would be greatly appreciated 🙂
Unless issues are found, I plan to merge and release this tomorrow.