resolver: include metadata in fetch

[Ratakor]

closes #60

Include metadata from pins.lock.json by default if pin.type = "fetch".
This mimics npins behavior to some degree.

I don't know if we should do something similar for "fixed" type. I don't think it's useful for "flake" type and it's kind of annoying to provide it by default.

[atagen]

it's a good start, but I'd really like to figure out something that works for fetchTree, FOD etc as well.

also still not sure I want the metadata riding on the input itself vs a _meta at the root.
if the sub-attribute may not be able to work that way for all fetchers, and may also be a forwards compatibility issue.
I definitely want something like this to be as consistent and opt-in as possible.

do you have any desire to expand the PR in this direction ? I'm happy to take up work on it, if not - it has been an open issue for a while.

[Ratakor]

I think it's fine if metadata is available at toplevel since sourceInfo is already present in flakes by default.