(PUP-3244) Allow autoload to handle missing environment

When a previous commit changed from get() to get!() the agent stopped
being able to use an environment that is available on the master, but
not available locally. I had misread the code and missed a trailing
&&. This caused me to think that the environment lookup in this case
was required, where in fact it is still optional.

This changes the structure of the code to make it much clearer that the
environment is optional, changes from get!() to get(), and adds some
tests around these code paths.