Use installed.json for extra discovery

[barryvdh]

As suggested @naderman in cb64747#commitcomment-22370108 this uses vendor/composer/installed.json, instead of looping through al package dirs.

Renamed the test to reflect the class, and cleaned up the packages.php file before running the test, to make sure it's fresh.

One note is that the order might change, but I'm not sure if we should assume it always just random?

[naderman]

Hah I wasn't even looking at the right file oh well ;-)

[barryvdh]

Taylor rewrote it a few commits later :) a5a0f3e

Hmm, tests are failing because the installed.json is not present. @naderman Should the file always be present, or is that a recent change?

[naderman]

@barryvdh it's been there since at least 2012: composer/composer@15ab586

[driesvints]

Sweet!