main.star: remove the concept of hosts

LUCI generally encourages builds to bind to hosts by dimensions, not by
name. Embrace that and get rid of our hosts by deriving dimensions from
the name.

There is a small problem: we have a concept of a scarce host. For now,
use the builder type minus any run mods to identify scarce hosts.
Long-term, it may get annoying to say that (e.g.) all Macs are scarce.
We'll see how it goes.

Change-Id: I73886da6fdfdf68196fdcd614f37890c6dae13cf
Reviewed-on: https://go-review.googlesource.com/c/build/+/512438
Auto-Submit: Heschi Kreinick <[email protected]>
TryBot-Bypass: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: heschi