Add support for importOnce

[szkrd]

Or something similar, in the veins of require_once from a popular language ;) If this absolutely is out of the scope, please feel free to close this ticket, otherwise it might be a useful placeholder. Thank you!

[arobson]

@rosamez, I'm not really familiar enough with PHP to understand the use case. How would that differ from the existing import? If you can explain the use case, I'd be happy to discuss it further.

[szkrd]

Import once would import a file only once, even if multiple importOnce has been issued. So if "class" B and C both depend on class A, then I still would be able to write
importOnce A in both files, but the file would only be included once.
Is this clear or does it sound silly?