This was a recent observation that slowed down local development on Impeller.

Making small changes to //impeller/renderer and re-building would seemingly
build all Impeller related targets (~650 actions). Even with Goma, this was
getting annoying. Turns out that //impeller/compiler was including
//impeller/runtime_stage that pulled in //impeller/renderer. It did so only for
a couple of headers.

The downside of this dependency is that a compiler invalidation rebuilds all
shaders and also the targets containing the generated C++ headers with shader
metadata. This causes the huge number of actions invocations.

Another downside is that the compiler was pulling in the renderer making it
larger and taking longer to link. Hilariously, it also pulled in Skia because of
the SkParagraph shaper used for text layout.

This patch reworks the dependency so that runtime stage enums are separate and
inverts the runtime_stage dependency on the compiler and renderer.

Now, editing and invalidating a //impeller/renderer or related target only
invokes < 5 actions instead of the ~650 earlier.