Fix collection items colliding when they share a basename

[nicodevs]

Closes #674

Issue

Two files in the same collection with the same basename (e.g. _posts/example.md and _posts/foo/example.md) would silently collapse into a single output. The collection is keyed by getFilename(), so the second item overwrites the first.

Solution

Items are now keyed by relativePath/filename when nested. the new method itemKey encapsulates the logic. User-facing access by basename ($siteData->posts->{name}) is unchanged for flat collections. For nested collections, two items that previously masked each other are now both reachable.