Need to trap and redirect the implementation of the import construct

[sparkprime]

There are contexts where allowing Jsonnet programs to access the file system is inappropriate, for example when a server executes code provided by a user. In those cases, it may still make sense for the import keyword to do something useful, but the service that is using the Jsonnet library needs to provide some sort of callback to handle the import construct in that case.

Jsonnet will still cache the result of the import to maintian referential transparency.