[6.x] Don't add per-site view paths when there's only one site

[lazerg]

The AddViewPaths middleware prefixes every view path and namespace hint with a /<site> sibling directory so a multi-site install can override templates per site. It does that on every front-end request, whether or not the install actually has more than one site.

On a single-site install those directories don't exist, so every prefixed entry is a guaranteed miss and the finder probes twice as many paths as it needs to. It's much worse on hosts that set open_basedir, since PHP can't cache a negative realpath lookup and each miss re-walks the whole path on every call. Counting the finder's file_exists() calls on a single-site install, a lookup went from 56 probes to 28 with the render unchanged.

This skips the middleware when there's only one site. Worth flagging: anyone on a one-site install who kept templates in resources/views/<site> would need to move them up a level. The issue also suggests filtering on is_dir() instead, which would help multi-site installs too, but that stats every path on every request, so I've kept this to the case that was reported.

Fixes #15323

[jasonvarga]

Fixed in #15325