Change ImportScriptPubKeys' internal to apply_label

Summary:
The internal bool was only to indicate whether the given label should
be applied as things that are internal should not have a label. To make
this clearer, we change internal to apply_label and invert its usage
so things that have labels set this to true in order to have their labels
applied.

This is a partial backport of Core PR16301 : https://github.com/bitcoin/bitcoin/pull/16301/commits/ab28e31c9563bd2cd1e4a088ffd2479517dc83f2

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6410