public-function-that-could-be-declared-as-external false positives

[elenadimitrova]

The following informational issues have started showing up with [email protected] when running against the argent-contracts repo:

INFO:Detectors:
0: isWhitelisted(address,address) should be declared external:
	- TransferManager.isWhitelisted(address,address) (modules/TransferManager.sol#381-385)
1: isValidSignature(bytes32,bytes) should be declared external:
	- TransferManager.isValidSignature(bytes32,bytes) (modules/TransferManager.sol#415-420)
2: isLimitDisabled(address) should be declared external:
	- LimitManager.isLimitDisabled(address) (modules/common/LimitManager.sol#129-132)
Reference: https://github.com/crytic/slither/wiki/Detector-Documentation#public-function-that-could-be-declared-as-external

All of the above are functions used internally in the same contract and so compilation fails if we make them external.