Avoid validation error when function parameter has internal function pointer

[ericglau]

Problem:
The following function
https://github.com/OpenZeppelin/openzeppelin-contracts-upgradeable/blob/e49c574f71d402bbd5905175ec2205af02e87dda/contracts/governance/utils/VotesUpgradeable.sol#L259-L262 causes the Upgrades plugin to give a validation error: Variable op is an internal function

This appears to be a false positive, since the internal function pointer comes from the same implementation code and is not kept in storage.

Background:
In #1032, we added validation errors for internal function pointers in storage variables.

This was done by using solidity-ast's findAll to look for all transitive nodes of type VariableDeclaration within a ContractDefinition.

The goal was to check all direct variables of a contract, or all variables within structs. But the contract's functions' parameters can also have VariableDeclaration type (which may be function pointers), and these are also transitive children of the contract -- these should be ignored.

Fix:
Instead of checking all transitive VariableDeclaration nodes from a contract, only check those from its direct variables or from its structs.

Fixes #1037