Generate composer.json

Summary:
JSON isn't the best candidate for template, but we can apply some
new high-order functions to the packages metadata to generate that.

Ref T1680.

Test Plan: make composer validate

Reviewers: dereckson, DorianWinty

Reviewed By: dereckson, DorianWinty

Maniphest Tasks: T1680

Differential Revision: https://devcentral.nasqueron.org/D2547