Use default values in pack.toml

[joelberkeley]

I want to use the latest commit of a dependency when building my project, so I'm using e.g.

[custom.all.elab-util]
type = "github"
url = "https://github.com/stefan-hoeck/idris2-elab-util"
ipkg = "elab-util.ipkg"
commit = "latest:main"

The only new thing there is commit = "latest:main", and it would be nice if pack could populate the other fields (type, url, ipkg) from its own toml (and error if the resulting config doesn't make sense). So that would look like

[custom.all.elab-util]
commit = "latest:main"

[stefan-hoeck]

Nice idea. I'll have a look but won't be able to do so before the weekend.