Adds much better support for the unstable Deno workspaces as well as
support for npm workspaces. npm workspaces is still lacking in that we
only install packages into the root node_modules folder. We'll make it
smarter over time in order for it to figure out when to add node_modules
folders within packages.

This includes a breaking change in config file resolution where we stop
searching for config files on the first found package.json unless it's
in a workspace. For the previous behaviour, the root deno.json needs to
be updated to be a workspace by adding `"workspace":
["./path-to-pkg-json-folder-goes-here"]`. See details in
denoland/deno_config#66

Closes denoland#24340
Closes denoland#24159
Closes denoland#24161
Closes denoland#22020
Closes denoland#18546
Closes denoland#16106
Closes denoland#24160