External hook implementation for fields/units and error messages

[awelzel]

@J-Gras found that the package template showed on Module::Unit { ... } usages in a comment and fixed it to explicitly use a ::%done as without that it's not functional: zeek/package-template@c429a20

Two topics related to that:

1. Should implicit %done work for external hooks on units as previously shown in the template, or was that an oversight? For unit fields %done is not needed, though that may be mixing different concepts that only on first sight seem related.

2. The error messages produced for hooks that have an extra %done on unit fields or a a missing %done on units fields are not helpful for diagnosing the issue.

Test files:

# test_mod.spicy
module test_mod;

public type Y = unit {
  y: bytes &size=1;
};
public type X = unit {
  x: bytes &size=1;
  y: 

# test_mod.spicy 
module zee_test_mod;

import test_mod;

# works
on test_mod::X::%done { }

# error: cannot use module 'test_mod' as an ID - could we say `%done` missing, or use `%done` implicitly?
# on test_mod::X { }

# works - is this implicit %done on the field or something else?
on test_mod::X::x { }

# error: unknown ID 'test_mod::X::b' - this is an extra %done where there shouldn't be one.
# on test_mod::X::b::%done { }

# works - is this implicit %done on the field?
on test_mod::X::y { }

# error: unknown ID 'test_mod::X::y' - same as for `::b` but with a unit type.
# on test_mod::X::y::%done { }

[rsmmr]

1. Should implicit %done work for external hooks on units as previously shown in the template, or was that an oversight?

Yes, it's meant to work. I think we ran into this issue before somewhere but looks like we don't track it in a ticket yet.

a missing %done on units fields are not helpful for diagnosing the issue.

One clarification: %done works only on units, not on unit fields. I guess one could say %done is always implicit on fields, but that we don't have %init on fields either, so that wouldn't be very consistent. But we can certainly improve error messaging here.