Performance impact of `PathBasedItem`

[mih]

This dataclass is used in pretty much all iterators of iter_collections. In #580 I saw that the conversion of path strings to PurePath instances can represent as much as half of the runtime of such an iterator.

It seems worthwhile to consider optimizations in this regard.

One possibility would be to change PathBasedItem to leave its name property untouched (Any), and add an accessor property (@cached_property) path that performs the conversion lazily. With this approach, any conversion performance cost would only need to be paid when actually necessary. Moreover, consumer would be given the choice of format of an item name.