This is a pretty fundamental change in Hugo, but absolutely needed if we should have any hope of getting "multiple outputs" done.
This commit's goal is to say:
* Every file target path is created by `createTargetPath`, i.e. one function for all.
* That function takes every page and site parameter into account, to avoid fragile string parsing to uglify etc. later on.
* The path creation logic has full test coverage.
* All permalinks, paginator URLs etc. are then built on top of that same logic.
Fixes#1252Fixes#2110Closes#2374Fixes#1885Fixes#3102Fixes#3179Fixes#1641Fixes#1989
The gain, given the "real sites benchmark" below, is obvious:
```
benchmark old ns/op new ns/op delta
BenchmarkHugo-4 14497594101 13084156335 -9.75%
benchmark old allocs new allocs delta
BenchmarkHugo-4 57404335 48282002 -15.89%
benchmark old bytes new bytes delta
BenchmarkHugo-4 9933505624 9721984424 -2.13%
```
Fixes#2495
