Improved package sorting

Hi,

While I was trying to resolve an issue that caused files defined by a same package to be scattered across the autoload files array, I noticed that the package sorting algorithm needed to be improved because the position (or importance) of the packages wasn't computed recursively and the resulting positions were sorted using `asort` which produced unpredictable results for packages with the same position, because as you know the sorting algorithm used by PHP is not stable (from the manual: If two members compare as equal, their relative order in the sorted array is undefined).

This new algorithm has two purpose: compute positions (or weights) recursively, so that a package required by a package of weight -10 has at least a weight of -11; use a stable sorting algorithm so that the order of packages with the same weight is preserved.

The following is a comparison of the current algorithm with the proposal algorithm. I'm using the [icybee/modules-nodes](https://packagist.org/packages/icybee/module-nodes) 2.x for testing purpose, because testing the package with the current Composer is causing a fatal error since the files of the [icanboogie/http](https://packagist.org/packages/icanboogie/http) package are loaded after the files of the [icanboogie/icanboogie](https://packagist.org/packages/icanboogie/icanboogie) package, although `icanboogie/icanboogie` requires `icanboogie/http`.

|PACKAGES                    |SORTED CURRENT                  |SORTED PROPOSAL                   |
|----------------------------|--------------------------------|----------------------------------|
|icanboogie/module-installer |icanboogie/module-installer (-7)|icanboogie/common (-81)           |
|ircmaxell/password-compat   |ircmaxell/password-compat (-6)  |icanboogie/inflector (-35)        |
|brickrouge/css-class-names  |brickrouge/css-class-names (-5) |icanboogie/prototype (-27)        |
|icanboogie/inflector        |icanboogie/inflector (-4)       |icanboogie/datetime (-24)         |
|icanboogie/common           |icanboogie/common (-3)          |icanboogie/event (-21)            |
|icanboogie/event            |icanboogie/event (-2)           |icanboogie/http (-13)             |
|icanboogie/datetime         |icanboogie/datetime (-1)        |icanboogie/errors (-9)            |
|icanboogie/prototype        |icanboogie/prototype (0)        |icanboogie/activerecord (-7)      |
|icanboogie/activerecord     |icanboogie/activerecord (1)     |icanboogie/routing (-6)           |
|icybee/module-users         |icanboogie/module (6)           |icanboogie/operation (-3)         |
|icybee/module-sites         |icanboogie/operation (6)        |icanboogie/module-installer (-2)  |
|icanboogie/errors           |icanboogie/routing (7)          |icanboogie/module (-2)            |
|icanboogie/module           |icanboogie/errors (7)           |ircmaxell/password-compat (-1)    |
|icanboogie/http             |icanboogie/icanboogie (7)       |brickrouge/css-class-names (-1)   |
|icanboogie/routing          |icanboogie/http (7)             |icanboogie/icanboogie (-1)        |
|icanboogie/operation        |icybee/module-users (9)         |icybee/module-users (0)           |
|icanboogie/icanboogie       |icybee/module-sites (10)        |icybee/module-sites (0)           |
|icybee/core                 |icybee/core (17)                |icybee/core (0)                   |

With this proposal, `icanboogie/common` is rightfully at the top since it is used by many packages, although sometimes indirectly; and `icanboogie/http` is well before `icanboogie/icanboogie` since it is used by `icanboogie/routing` and `icanboogie/icanboogie` and by other packages as well although indirectly. Also notice that the order of packages with the same weight is preserved.

Note: This pull request is similar to the [PR #2642](https://github.com/composer/composer/pull/2642), but it features only the diff for the `sortPackageMap()` mehtod.