Dataflow tracking runs the installPhase of a derivation, and then
does its best to track build input -> store path output pathways.
This allows for substituting the installPhase of a derivation as much
as possible with relatively quick store path logic, allowing for cross-
project build input tracking.
Change-Id: I6a6a69642530b32edbc2c521a3f584703731b6e1
Incremental builds now take depfiles in account! Using a file that
contains a mapping of depfile to its cached contents, Zilch now
rebuilds less targets unnecessarily! If a build fails because an
include is added, it transparently rebuilds it with all possible header
dependencies, using the custom build scheduler built into Zilch. If an
include is removed, the target will be rebuilt with the new set of
headers the next time the CLI is invoked.
Change-Id: I6a6a6964c2fb191af4a474c45fd0f29623c588b0
