Replace InputRule with Path.

Summary:
Updated `ConcurrentMapFileHashCache` to use a `LoadingCache` to maintain the
mapping from `Path` to `HashCode`.

Previously, `InputRule` contained four fields (`File`, `BuildTarget`, `ListenableFuture`, `RuleKey`).
We had one of these for potentially every file in a project. That is a large potential for
wasted memory. The existence of `InputRule` also required us to persist some kludginess in
`BuildTarget`.

This diff removes the hacks from `BuildTarget` and reclaims potentially a ton of memory.
By using a `LoadingCache`, we do not have to worry about the same input file being sha1'd
multiple times. We can also stop passing around references to a pathResolver in many places
and rely on `Path` being a path relative to the project root.

Test Plan: Sandcastle builds.