Decide how to distinguish between core externs from user-provided externs

[asl]

Now I'm confused whether we can tell a user-defined packet_in extern like this, from a packet_in extern from core.p4? I guess we can't?

Then I wonder how could we treat packet_in from core.p4 differently as discussed in #93?

Originally posted by @qobilidop in #94 (comment)

[asl]

Currently we're also having a precedent of lowering verify calls during the translation. I would probably do this in two-step fashion:

• Translate stuff as-is (so we will have original P4 semantics with externs, etc.)
• Lower core methods in a second step

We might want to use some kind of @core or so annotation for this builtins defined in the spec. I can imagine downstream users might want to do something like this for their own externs, etc. with special semantics.

[asl]

Annotation was submitted in p4lang/p4c#5285