refactor asset identification (#3999)
* add Asset::ident() as unique identifier of an Asset
* add ChunkItem::ident() instead of ChunkItem::to_string()
(ValueToString)
* base `chunk_path` on AssetIdent instead of path only
Motivation:
We want to get rid of the `import "."` in favor of inner assets. When
doing this we no longer need to place virtual assets below the actual
file path and they can stay in their original path. But placing virtual
assets below the actual asset also made the `Asset::path` unique, which
would no longer be the case after using inner assets. Some parts of the
code base relied on `Asset::path` being unique (e. g. module ids and
chunk paths). But actually we never guaranteed that to be unique.
After this PR `Asset::ident` is intended to be unique and allow to carry
more information than only the path:
* Query string (`module?query`)
* Fragment (`module#fragment`)
* Asset (additional wrapped assets by key value pairs)
* Modifiers (additional transformations applied on the module, e. g.
`chunks`, `client chunks`)
* In future: Part (select a subpart of the module, e. g. only export
abc, or the module evaluation, or some internal part)
