Improve the freeze/extract system

[Georges Racinet]

Currently, the freeze and extract features are provided by options, because that seemed simpler to implement, in a piece of code that has all the necessary information. But this has several drawbacks

- it is somewhat counter-intuitive
- the build is performed again, which jeopardizes the very idea that a release manager should freeze exactly what has been locally validated. In particular, even though --offline is forced by the freeze-to command, there are known cases where the freeze can have a different Python dependency version (if a newer one is present in a shared eggs directory), and several subtleties with VCS addons

A better system:

- improve contents of .installed.cfg
- provide separate executables (bin/freeze and bin/extract_downloads) that use .installed.cfg

No matter what, VCS content should be what is locally present, not a blind copy of what should be present. Command-line options for the new scripts can be provided in order to control the behavior. For example, a flag to allow using the local version if different from the configuration file (by default refuse a discrepancy).