archlinux: use pacman -T to verify installation

[de-vri-es]

Currently rosdep uses pacman -Q to check if a package is installed. However, that does not work with targets provided by a package of a different name.

For example: the eigen package also provides the eigen3 target. Running pacman -Q eigen3 will exit non-zero though, because there is no package named eigen3 installed. Running pacman -T eigen3 will exit with status 0, because the target eigen3 is provided by an installed package.

This PR changes rosdep to use pacman -T.

pacman -T prints the non-installed packages, so it could even be used to implement pacman_detect with a single pacman invocation. I didn't think that was worth the added complexity over the current implementation though.

[tfoote]

This looks reasonable to me, based on the basic documentation.

Are there any Arch users following this who could test?

[hgaiser]

Running pacman -Q eigen3 does indeed return with exit status 1. Running pacman -T eigen3 returns with exit status 0 (and just to verify, running pacman -T eigen33 again returns with exit status 1).

[de-vri-es]

I also tested locally. Note that the current behaviour is broken when installing the rosdep key eigen, since it resolves to the package eigen3 on Arch Linux. While in this case it may have been better to have the rosdep key eigen resolve to the eigen package directly, in general it should be possible to have rosdep keys resolve to virtual targets.