refactor: Parse BUILD-files once

[tobni]

Was a theoretical cold-start performance gain that turned out to be minor. I do think this is cleaner though.

[cburroughs]

Thanks for looking at this and the multi-parse has bothered me every time I looked at it.

I'm struggling to find my prior notes, but when I tried this it was counterintuitively slower on whatever I tried it with, presumably because ast.parse(str)+ast.compile(tree) doesn't just to hang out in C like ast.compile(str). Was it noise or faster with what you were looking at?

[tobni]

This change is within noise wallclock but does reduce total cpu, mostly due to reduced allocations. It scales with number of build files and symbols therein.