Fix get reviewers' bug (#32415)

This PR rewrites `GetReviewer` function and move it to service layer.

Reviewers should not be watchers, so that this PR removed all watchers
from reviewers. When the repository is under an organization, the pull
request unit read permission will be checked to resolve the bug of
#32394

Fix #32394

tests/integration/api_repo_test.go

@@ -718,8 +718,8 @@ func TestAPIRepoGetReviewers(t *testing.T) {
 	resp := MakeRequest(t, req, http.StatusOK)
 	var reviewers []*api.User
 	DecodeJSON(t, resp, &reviewers)
-	if assert.Len(t, reviewers, 3) {
-		assert.ElementsMatch(t, []int64{1, 4, 11}, []int64{reviewers[0].ID, reviewers[1].ID, reviewers[2].ID})
+	if assert.Len(t, reviewers, 1) {
+		assert.ElementsMatch(t, []int64{2}, []int64{reviewers[0].ID})
 	}
 }