Add support for all Linux distros using apt

[seanballais]

Fixes #7307

Changelog: Feature: Checking if a Linux distro uses apt is now based on the existence of apt in the system, instead of checking if the distro currently being used is in a hard-coded list of distros known to use apt.
Docs: omit

[CLAassistant]

All committers have signed the CLA.

[memsharded]

I would say that it is extremely unlikely that some distro will have other thing different to apt, located in /usr/bin/apt.

What I am not 100% sure is that all those different distros will have apt located in that exact place, and not elsewhere. Have you checked that?

Thanks for contributing this!

[SSE4]

@seanballais I am also not sure, may apt reside for instance in directories like in /bin, /usr/local/bin, /opt, etc? maybe using tools.which and then checking some apt-specific command may run (such as apt-get or apt-cache).
I personally don't like hard-coded paths, and can imagine edge cases where apt was installed from sources to the non-standard directory.

[seanballais]

@SSE4, @memsharded, I updated the check based on your feedback. I detracted though a bit, and checked for apt-get itself instead since it it was AptTool uses (aside from apt-add-repository, etc.). As a final check, I checked if the help text of the detected apt-get has "This APT has Super Cow Powers.", which is in the actual apt-get's help text. The only problem I have with this check is that a modified or future official version of apt package is used where the aforementioned substring is removed. Should we worry about this problem? Anyhow, I'd like to get your feedback.

[SSE4]

@seanballais yes, this definitely might be a problem, I suspect text might be different for different locales, e.g. it could display Spanish translation for es-es, etc.:

apt-get --help
apt 0.7.14ubuntu6 для i386 скомпилирован Aug 14 2008 16:54:20
…
В APT есть коровья СУПЕРСИЛА.

just running something like apt-get moo and checking that exit code is zero should be way more reliable

[seanballais]

@SSE4, I've just changed the check from using --help to using moo.