Adding dependency on non-existing module hi-jacks admin/build/modules

When adding an arbitrary dependency on a non-existing module, like so:

dependencies[] = unleashed

you get:

1) plenty of PHP notices on admin/build/modules

2) a new package container with an empty module row that says "This version is incompatible with the 7.0-dev version of Drupal core."

3) an empty "Requires: (disabled)" for the module that requires the non-existing module

Attached patch (by chx) fixes 1) and 2).

However, 3) gets worse: The "Requires: " is not output at all and the module defining the requirement can be enabled.

Lastly, we need tests for this.