Clarify use of underscores in HACKING

[Ian Clatworthy]

This patch is a small change to HACKING to better reflect the convention
used around underscores. I had been using an underscore to mean
"internal to this module" and a lack thereof to mean "external to this
module" when Robert explained to me that underscore really means
"internal to bzrlib".

As that was a partial surprise, I think we need to make that clearer in
HACKING.

Ian C.
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