Quick question about disabled integrity tag in selection.d

[Quang Ha]

Hi everyone,

I was reading `selection.d` in dub and saw that `toJsonDep()` has 
`version (none)` around the integrity tag:

"The following is not yet enabled, because we're currently only 
able to get an integrity tag value when the package is first 
downloaded..."

Because hash is only got on first download, it flips between 
empty or not when reusing cached packages.

If I work on saving this hash in the local cache so dub always 
remembers it, would @s-ludwig and the team welcome a PR for this?

Thanks!