make Target cacheable in the ProductGraph

[kwlzn]

currently, BuildGraph construction is responsible for the ~majority of our runtime in the warm case. much of the cost here lies in Target instantiation which happens on each run by way of converting cached LegacyTarget/TargetAdaptor objects.

if we can instead construct Target objects directly in the daemon+engine context and thus make them cacheable in the resident ProductGraph, then we can theoretically skirt their construction costs on warm runs and simply link them together within a per-runBuildGraph.

in order to do this, we'll probably have to:

1. kill all mutable state on Target
2. remove Target->BuildGraph linkages

[stuhood]

If the warm performance is "good enough" for now, I suspect that focusing on #4213 and #3560 would be more holistic fixes. I'm going to drop this from the milestones.

[kwlzn]

sgtm.

[stuhood]

It seems likely that this should be dropped entirely in favor of #4769, which proposes to not create a BuildGraph at all for frontend Task instances that have been updated to have product requirements. That would mean that those tasks would immediately begin consuming the v2 products that we're exposing, so they would need a lot more polish via #4535.

[stuhood]

I'm going to close this. I believe that #4535 and #4769 invalidate it.