Improve error message for bound typevar in TypeAliasType

[hamdanal]

Follow up to #17038

When a type variable is bound to a class, it cannot be reused in a type alias. Previously in TypeAliasType, this error was reported as "not included in type_params". However in the following example, the error is misleading:

from typing import Dict, Generic, TypeVar
from typing_extensions import TypeAliasType

T = TypeVar("T")


class A(Generic[T]):
    Ta11 = TypeAliasType("Ta11", Dict[str, T], type_params=(T,))


x: A.Ta11 = {"a": 1}
reveal_type(x)

On the master branch:

main.py:8: error: Type variable "T" is not included in type_params  [valid-type]
main.py:8: error: "T" is a type variable and only valid in type context  [misc]
main.py:8: error: Free type variable expected in type_params argument to TypeAliasType  [type-var]
main.py:12: note: Revealed type is "builtins.dict[builtins.str, Any]"
Found 3 errors in 1 file (checked 1 source file)

With this PR:

typealiastype.py:8: error: Can't use bound type variable "T" to define generic alias  [valid-type]
typealiastype.py:8: error: "T" is a type variable and only valid in type context  [misc]
typealiastype.py:12: note: Revealed type is "builtins.dict[builtins.str, Any]"
Found 2 errors in 1 file (checked 1 source file)

This is possible by storing the names of all the declared type_params, even those that are invalid, and checking if the offending type variables are in the list.

[hamdanal]

It would be nice to also get rid of the second remaining error, but this will require adding a special-cased context only for analyzing type_params and I am not comfortable doing this change without first consulting someone with more experience than me.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅