stubtest: emit error if a stub defines a public type alias that doesn't exist at runtime

[AlexWaygood]

Description

A followup to python/typeshed#7634 (comment)

Stubtest currently declines to error if a stub defines a public type alias that doesn't exist at runtime. But it should!

These are the new error reported from running stubtest on the typeshed stdlib, with this patch applied:

error: _threading_local.localdict is not present at runtime
Stub: at line 6
Type alias for: builtins.dict
Runtime:
MISSING
error: audioop.AdpcmState is not present at runtime
Stub: at line 3
Type alias for: Tuple[builtins.int, builtins.int]
Runtime:
MISSING
error: audioop.RatecvState is not present at runtime
Stub: at line 4
Type alias for: Tuple[builtins.int, builtins.tuple[Tuple[builtins.int, builtins.int], ...]]
Runtime:
MISSING
error: itertools.Predicate is not present at runtime
Stub: at line 14
Type alias for: def (_T?) -> builtins.object
Runtime:
MISSING
error: sqlite3.dbapi2.OptimizedUnicode is not present at runtime
Stub: at line 413
Type alias for: builtins.str
Runtime:
MISSING

They all look like true positives to me (or at least should be allowlisted in typeshed, rather than being ignored by stubtest altogether).

Test Plan

Two test cases added

[hauntsaninja]

Thanks! I think it can be easier to preserve line numbers... I took the liberty of pushing changes directly to the branch, let me know what you think!

[AlexWaygood]

I think it can be easier to preserve line numbers... I took the liberty of pushing changes directly to the branch, let me know what you think!

Nice, thank you!! That's... much simpler than my method 🤦‍♂️

Just one nit about error messages -- we currently have:

Stub: at line 4
Type alias for Tuple[builtins.int, builtins.tuple[Tuple[builtins.int, builtins.int], ...]]

I'd much prefer either this, separating the English from the code using a colon:

Stub: at line 4
Type alias for: Tuple[builtins.int, builtins.tuple[Tuple[builtins.int, builtins.int], ...]]

or this, separating the English from the code using quotation marks:

Stub: at line 4
Type alias for "Tuple[builtins.int, builtins.tuple[Tuple[builtins.int, builtins.int], ...]]"

What do you think?

[AlexWaygood]

I approve my own PR

[AlexWaygood]

Thanks @hauntsaninja :)