Adding the "build in branch" functionality directly into bilt

[giltayar]

The "dev" directory has scripts that can install a local registry that aids in local development and in branches. We can incorporate this into Bilt (probably using a separate CLI?)

Something like this:

bilt-local start @bilt

...to start the local npm registry and have the @bilt scope publish to it. And this:

bilt-local list

...to list of all scopes running under a local npm registry. And finally, this:

bilt-local stop @bilt

...to remove the local registry and have the @bilt scope point to the previous registry.

[giltayar]

After my experience working with branches and bilt, it is now my conviction that the way Bilt figures out whether a package was built or not is wrong, and that currently the only way to use Bilt is to do trunk-based development without PRs.

So closing the issue till I figure out how to incorporate PR-based workflows into bilt.