Serdes v2: Rebuild the directory structure to be more human-friendly (#26793)

There are now three top-level trees:

- regular `collections/path/to/collection/...`
- `:namespace :snippet` collections in `snippets/path/to/collection/...`
- `databases/mydb/schemas/PUBLIC/tables/customers/fields/name.yaml`

The path for any given entity is determined by the
`serdes.base/storage-path` multimethod.

On the ingestion side, things are a bit tricky because the paths don't
map directly to `:serdes/meta` hierarchies anymore. Instead each model
registers a function to turn a file path into either a `:serdes/meta`
hierarcy or nil for a bad match.

Ingestion will fetch all these functions once and then try them all in
arbitrary order until one matches.