feat!: require compound functions names in extension references

[vbarua]

BREAKING CHANGE: plans referencing functions using simple names (e.g. not vs not:bool) will no longer be valid.

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[vbarua]

As discussed in Slack, this PR removes the ability to use simple names in function references. This change makes function lookup easier from an implementor perspective.

It also helps avoid a potential pitfall where, for a function with only a single implementation, adding an implementation becomes a breaking change as an plan that referenced that function using its simple name is no longer valid (as the function reference is now ambiguous).

[jacques-n]

I'm good with this change

[vbarua]

@westonpace Added a BREAKING CHANGE to the commit message. The feat! should also mark this as a breaking change. Is that what you had in mind, or is there something else I need to do?

[mbrobbel]

@mbrobbel we might need to update the validator to be more strict now (and can probably simplify some things in the process). Let me know if I can help.

Yes, this being more strict simplifies the validation logic. I haven't had much time lately to make progress on the parser, but I do plan on continuing the effort. I already have a TODO for this, because I didn't implement compound name parsing yet.

[westonpace]

Thanks for writing up this change!