Separate build & deploy so that each can be optimized (parallelism, etc) independently

Build and deploy are two distinct phases of resource management and should be treated as such; doing so would let tilt make optimizations specific to each phase.

Right now tilt cannot parallelize docker builds of images that share the same root image and are on the same level in the dependency tree and have multiple k8s resources that use the images. If we split build into its own phase, the docker image dependency tree can be parallelized as needed at any level in the tree, significantly speeding up initial startup and rebuild in some tilt deployments.

We are running into this in our tilt deployment - we have a base image that has a tree of dependencies a few levels deep with a bunch of neighboring images at each level in the tree. With current tilt this all builds in serial and takes 10-15 minutes just to build; if we could do the build in parallel it would shave large amounts of time off our overall startup time.

No files, tests, or entry points are named. Start by tracing how Tilt currently schedules image builds and Kubernetes deployment, then determine the boundaries needed to optimize the phases independently. Done means build dependencies can be parallelized without being constrained by deployment resources.