Split into fetch-psc-packages and just-build parts

[robinp]

Sorry if this is already the case. Using psc-packages with success to build my project, but now I'm wrapping it up as a Nix expression, and it would help to be able to separate in two steps:

1. Only creates .psc-package based on the psc-package.json file. Fetches from git and stuff. Doesn't build.

2. Only builds, doesn't try to access network or in other way update .psc-package.

This would let me effectively cache 1), and rerun 2) relatively fast.

1. might already be satisfied by some command (install? update?), but I'm not sure. For 2), psc-package build seems to try to check the updates (though I'm on a former version from stable Nix channel).

Thank you!

[nwolverson]

A similar pattern tends to be useful for leveraging docker layer caching