Allow package.js to have dev dependencies

[vparpoil]

When writing atmosphere packages, you may want to use dev-only functionalities. These functionalities should not be compiled for production by the meteor build command.

package.js file should allow to define specific Api.use or Api.addFiles that would be compiled only in development Mode (ie when Meteor.isDevelopment === true). It could be done in another definition in Package like Package.onDev.

The use case is similar to Npm devDependencies.

Alternative for us for now is to use conditional imports inside the package.

What do you think ?