Check_for_package method should not use which command

[PierreRambaud]

For Unix::pkg, the method check_for_package is wrong.
Currently, the method check for a program instead of a package.

For example:
If we install libxml2-dev on a Debian os, we can't use check_for_package(master, 'libxml2-dev')

The real command to checking if a package is installed on a debian|ubuntu os is dpkg -s #{name}.
For others, I don't know a good command without grep or regex to check if package is installed.

[anodelman]

Yes, the wording here is definitely wrong. I am going to put a version of this bug into the internal bug tracker and start working on a patch.

[anodelman]

In PR,

#229

[PierreRambaud]

Thanks ;)