Improve TupleStruct ctor privacy err diagnostic when working cross-crate

[guswynn]

currently
#78401
and
#76499

only work with types within the crate, as the struct_constructors only has full info for types within the current Crate being built
We can likely recover this info in the extern crate metadtata, but as reaching into this metadata is likely slow and complicated, this should only happen on the err path

I will see if I can find time to work on this

cc @estebank @petrochenkov

@rustbot modify labels to +T-lang, +A-diagnostics

[guswynn]

@rustbot modify labels to +T-compiler, -T-lang

sorry!