hugolib: Speed up GetPage

hugolib: Speed up GetPage

When we know to look into the index pages collection, do that:

```
benchmark              old ns/op     new ns/op     delta
BenchmarkGetPage-4     51483         7072          -86.26%

benchmark              old allocs     new allocs     delta
BenchmarkGetPage-4     71             71             +0.00%

benchmark              old bytes     new bytes     delta
BenchmarkGetPage-4     2648          2648          +0.00%
```

This commit also returns an error if .Site.GetPage is called with the regular Page Kind, as that is currently not supported.

Fixes #3503