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
 		}

internal/courier/smtp_test.go

@@ -58,7 +58,7 @@ func TestSMTP(t *testing.T) {
 	// Put a non-existing host first, so we check that if the first host
 	// doesn't work, we try with the rest.
 	// The host we use is invalid, to avoid having to do an actual network
-	// lookup whick makes the test more hermetic. This is a hack, ideally we
+	// lookup which makes the test more hermetic. This is a hack, ideally we
 	// would be able to override the default resolver, but Go does not
 	// implement that yet.
 	testMX["to"] = []*net.MX{

internal/set/set_test.go

@@ -13,10 +13,10 @@ func TestString(t *testing.T) {
 	s1.Add("a")
 	s1.Add("b", "ccc")
 
-	expectStrings(s1, []string{"a", "b", "ccc"}, []string{"notin"}, t)
+	expectStrings(s1, []string{"a", "b", "ccc"}, []string{"not-in"}, t)
 
 	s2 := NewString("a", "b", "c")
-	expectStrings(s2, []string{"a", "b", "c"}, []string{"notin"}, t)
+	expectStrings(s2, []string{"a", "b", "c"}, []string{"not-in"}, t)
 
 	// Test that Has works (and not panics) on a nil set.
 	var s3 *String