feat: Implement automatic CI target detection for the depot r/1725

Automatically walk the entire depot tree and pick out things that are
"buildable", then include them in the attribute `ci.targets` (which is
now also the target for CI builds).

A long time ago, in a land far away, we (well, I, at the time) had a
prototype of this which ran into constant issues with infinite
recursions while trying to walk the tree. In fact, this is why
readTree originally gained the `__readTree`-attribute which marks
things that were imported automatically.

Based on some code edef whipped up earlier (with the breakthrough
being that we also add the attribute to top-level folders, which
suddenly resolves a whole bunch of problems), I've now implemented
this actually working version.

At the moment all builds still happen as one big bag of builds, but at
some point we will granularise this.

Change-Id: I86f12ce7f63dae98e7e5c6646a4e9d220de783f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1854
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Reviewed-by: glittershark <grfn@gws.fyi>

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