hp-check: Add constants for the dependency indices

[Francois Gouget]

check.py and core_installer.py use literal constants such as 3, 5 or 7 to access the fields of the dependencies array. This results in very error-prone, obfuscated and unmaintainable code.

So I introduced constants: DEPENDENCY_VER_MIN_INDEX is a lot clearer than '3'.
(I'll grant you that it's a bit verbose but to my defense I only followed the existing naming convention.)

See the attached patch. This patch can also be found on:
https://github.com/fgouget/hplip

[Francois Gouget (fgouget)]

More precisely it is on the fixes branch:
https://github.com/fgouget/hplip/commits/fixes