Deferred build script execution

[jeffparsons]

Right now we run build scripts or pull their output because we don't know if we'll need their output to compile the main crate, or if we'll be able to pull it.

We could instead pretend to run the build script by just emitting the stdout/stderr we find in cache, and then only actually run it if we need to from within the rustc wrapper.

[jeffparsons]

Changed direction on this a bit... not going to bother writing the whole thing up now. TL;DR is:

• Always put a copy of hope in place of the build script.
• Decide whether we actually need to run the build script, and do run it immediately (not later when building the main crate) if needed.
• Fake '.d' output.

But... I think I also need to filter the build script output, not just for paths, because otherwise it will decide that it needs to rerun all the time because its output is never present in the out dir if I'm pulling from cache!

So that's what this ticket now is.