Package builds

[guybedford]

WIth jspm install, you can install a file or an entire package via jspm install preact for just the main or jspm install preact/ for all exports.

Similarly for the build, we could enable package builds via this trailing slash, as it extends the same meaning of building all exports instead of just the main.

In addition we can infer the trailing slash when pointing to a directory to build anyway.

The output would include all entry points as well as a built package.json file for the entire built package.

This build is then exactly the local build, and should eventually be the same build used by the jspm install --verify system as well.

Extending package builds to multiple package builds, is then the multi monorepo build use case.