Allow `importMap` field in child workspaces/packages

[CyanChanges]

Currently, if you specify a importMap in a child workspace(/package), it gives your a warning like:

Warning "importMap" field can only be specified in the workspace root deno.json file.
    at file:///home/cyan/WebstormProjects/koishi-registry-migrate/krts/packages/terminal/deno.json

And it doesn't work.

I think it is good to allow importMap in child workspace(/package)s,
just like how specify a imports field works.