helpers: Avoid url.Parse in RelURL and AbsURL if we can

helpers: Avoid url.Parse in RelURL and AbsURL if we can

```
RelURL-10            159ns ± 5%      18ns ± 4%   -88.89%  (p=0.029 n=4+4)
AbsURL/relurl-10     532ns ± 0%     537ns ± 0%    +1.09%  (p=0.029 n=4+4)
AbsURL/absurl-10     142ns ± 0%       4ns ± 3%   -96.91%  (p=0.029 n=4+4)

name              old alloc/op   new alloc/op   delta
RelURL-10             144B ± 0%        0B       -100.00%  (p=0.029 n=4+4)
AbsURL/relurl-10      544B ± 0%      544B ± 0%      ~     (all equal)
AbsURL/absurl-10      144B ± 0%        0B       -100.00%  (p=0.029 n=4+4)

name              old allocs/op  new allocs/op  delta
RelURL-10             1.00 ± 0%      0.00       -100.00%  (p=0.029 n=4+4)
AbsURL/relurl-10      10.0 ± 0%      10.0 ± 0%      ~     (all equal)
AbsURL/absurl-10      1.00 ± 0%      0.00       -100.00%  (p=0.029 n=4+4)
```