Raise clearer exception when using a yet undeclared variable in a type annotation

[benber86]

What I did

As per #3214, trying to reference a yet undeclared constant variable in the type annotation of another variable's declaration results in a fairly unhelpful error message: vyper.exceptions.UndeclaredDefinition: 'self' has not been declared.
This fix ensures that a more detailed exception is raised.

How I did it

The exception is raised when trying to check whether the variable is a storage variable by checking if its name is in self.namespace["self"].typ.members.
However this check can happen before self is added to the namespace. When the constant variable is referenced in a type annotation, the check will happen during folding and before validation, so there will be no self key in namespace, leading to the previous exception being raised.
A simple check on whether the key is present suffices to prevent this exception from being raised and to instead raise another detailed UndeclaredDefinition exception containing the undeclared variable's name and line number of the error.

How to verify it

Compiling the following contract:

PRECISIONS: constant(uint256[N_COINS]) = [
    1000000000000,
    10000000000,
    1,
]
N_COINS: constant(int128) = 3

Now raises the following exception:

vyper.exceptions.UndeclaredDefinition: 'N_COINS' has not been declared. 
  contract "***.vy:1", line 1:29 
  ---> 1 PRECISIONS: constant(uint256[N_COINS]) = [
  ------------------------------------^
       2     1000000000000,

Commit message

Raise clearer exception when using a yet undeclared variable in a type annotation

Description for the changelog

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