no way to express build dependencies in Tilt

Currently, Tilt has resource_deps for expressing runtime initialization dependencies (e.g., "You need to start up the database before the frontend").

But Tilt has no way to express other types of dependencies, including:

1. A common pattern is to have a local_resource that generates files, and then a docker_build that syncs those files to a container. (e.g., generating API documentation in tilt.build, compiling Go static binaries). This workflow has a bunch of rough edges (logs aren't grouped like you would expect them, redundant builds, etc)

2. Another common pattern is to have a local_resource that solely exists to kick off another build (e.g., the "touch Tiltfile" script described in https://github.com/windmilleng/tilt/issues/2957, or the "measure deployment" script described in https://docs.tilt.dev/example_static_html.html). These scripts are prone to race conditions and infinite loop problems (https://github.com/windmilleng/tilt/issues/1930)

People often try to do this with resource_deps, and are confused about why it doesn't work. See also discussion on this PR: https://github.com/windmilleng/tilt/pull/2837#issuecomment-578315808

I don't know what the solution is to this problem, or even if (1) or (2) are the same problem. This issue is just to have a place to point people who ask about this while we think about what to do about it.