Proposal: Add pre build validation hooks

[renoirb]

When exposing build step, provide available module lookup before running build.

Say we provide a system to bundle a package, that it reads babel/env, Browserslist, Core-JS, etc. And want to help the author make variants of the build for different build targets. But the build step run on host package lacks dependencies. The build likely will have warnings but won't necessarily fail. But the output would definitely be different.

I've worked on extending bili, and would propose to port my ideas here.

For example:

• how to ensure core-js exists and how to configure bili with it (implementation, published on NPM)