`panic` when asked for a module in `ExternalDeclProvider`
Summary:
While reviewing the previous stack w/ jakebailey he noted that I probably ought to panic here rather than always state that modules don't exist.
My initial reasoning was defensive: HackC crashes are *bad* as they're difficult to surface to users. It's technically correct to say modules never exist in this position as they (currently) aren't exposed to the runtime
Jake's reasoning was that in the event that we *do* expose modules to the runtime, and they're asked for in this position, it's going to be confusing to debug if they're all not found. Additionally, asking for modules *at all* until that time is definitely a mistake. This fails loudly rather than silently.
Differential Revision:
D34940879
fbshipit-source-id:
d424e5d621907883b1570fcf5b707af096fa448c