Only public aliases should be taken account when diffing

[stevapple]

In practice we (and many of the Python users/communities) consider underscore-prefixed members and modules (i.e. whose identifier starts with exactly one _) to be unstable, private or internal implementation detail, which will not be documented and API stability is not guarenteed.

It would be great to provide an option/tool to exclude underscored members from extract results.

[stevapple]

After some investigation I found the real cause might be with moving an exported private member arount. aexpy marks the internal symbol itself as public when it's exported, but it's not true because the real API exported is its alias. This will give false positives when the internal layout changed while the API remains the same.