errors: move similarity_hint() to error module I want to be able to reuse it from `UnknownIdentifier`'s constructor. Moving it results in a new import of `difflib` in the `error` module. There was a comment at the top of `error.py` saying "Do not import anything but pycompat here, please", which was added (except for the "pycompat" bit) in 08cabecfa8a8 (errors: move revlog errors, 2009-01-11). I don't know the reason for the comment. I'm guessing the point was to not make the module depend on other Mercurial modules. If that was it, then importing `difflib` should be fine. Sorry about the churn (I moved this code from the `dispatch` module to the `scmutil` module very recently). Differential Revision: https://phab.mercurial-scm.org/D9345

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