Fix minor typos found by codespell

This patch fixes some minor typos in comments and strings found by
codespell.

While at it, also expand some variable names that were not typos, but
caused false positives, and end up being more readable anyway.

internal/aliases/aliases.go

@@ -271,8 +271,8 @@ func (v *Resolver) resolve(rcount int, addr string, tr *trace.Trace) ([]Recipien
 			// aliases).
 			// The queue will attempt delivery against this local (but
 			// evidently non-existing) address, and the courier will emit a
-			// clearer failure, re-using the existing codepaths and
-			// simplifying the logic.
+			// clearer failure, reusing the existing codepaths and simplifying
+			// the logic.
 			tr.Debugf("%d| no catch-all, returning %q", rcount, addr)
 			return []Recipient{{addr, EMAIL}}, nil
 		}