Make scriptlet validation even safer

[bensmrs]

Currently, we validate scriptlets simply if they pass Starlark’s parsing. I intend to add an additional validation step in the *Validate methods to ensure that required functions (qemu_hook for QEMU, instance_placement for InstancePlacement, authorize for the future Authorize) are actually defined in the scriptlet.
It will not ensure that the scriptlet works or is well-typed, but adds quite a bit of safety.

If it looks good to you, you can assign me on the issue and I’ll ship it with the Authorization scriptlet.

[stgraber]

Sounds good!