[RFC] virtual env. like.

[liuggio]

A very common and useful command especially in python is changing environment,
it is used also to see how the application reacts with different versions of dependencies.

It would be very useful to be able to switch 'env' in composer, especially during debugging and testing.

I see 2 solutions for doing this:

A require section for each env.

 "require-sf2.4":  { ...

Or adding a new json file called composer-{env} containing only the differences with the original composer file.

Do you really think that debugging and testing are weaks use cases?

[liuggio]

found another use case #2129, different application env. with different dependecies.

[Seldaek]

This should be possible to handle with the COMPOSER env var (which loads a new composer.json) and #1013 for extending the file.