Only consider errors from modifiers in use

[ericglau]

Related to #702

Modifiers with unsafe opcodes in parent contracts are reported, but even if those modifiers are not used. This is because in https://github.com/OpenZeppelin/openzeppelin-upgrades/blob/master/packages/core/src/validate/run.ts, getDirectFunctionOpcodeErrors finds the modifier's FunctionCall containing the unsafe opcode, although the NodeType is a ModifierDefinition rather than a FunctionDefinition. Therefore isInternalFunction evaluates to false since this is not a FunctionDefinition, and the check is not skipped.

Instead, the above should skip modifiers (which are internal), but getReferencedFunctionOpcodeErrors should also look for ModifierInvocation nodes and dereference those.