hplip: prevent conflicts with the GUI package

[ahesford]

The hplip-gui package uses the exact same files as the regular hplip package, plus a few extra for the UI and three changes in the configuration file. Rather than copy the entire package contents and register conflict= and provides= to fool the package manager, we can make hplip-gui depend on hplip and just manage a symlink between two alternative configuration files.

The only really sophisticated part is handling the configuration symlink when removing hplip-gui. Because a user might remove the GUI package and want to keep the non-GUI version installed, the remove hook should detect whether a) it owns the existing symlink (i.e., the link points at the GUI configuration) and b) whether the non-GUI configuration exists (in which case the link should be moved to the non-GUI config; otherwise, it should be removed). The rest of the hooks are very basic:

• hplip only claims the configuration symlink on installation if no valid configuration symlink exists, to avoid clobbering, e.g., the existing GUI link on an upgrade.
• hplip-gui claims any configuration symlink (but leaves regular files alone) because a user installing the GUI package probably wants it to work.
• hplip just removes any configuration symlink it owns when the package is removed.

The removal hooks also treat a failure of readlink (which really ought to never happen) as if they "own" the symlink.

I've run through a few installation and removal cycles, and this seems to do what I expect.

[ahesford]

I moved every usr/bin/hp* strictly-GUI application and its corresponding script in usr/share/hplip to the hplip-gui package. These are clearly intended to be __main__ entrypoints, so their absence from the hplip packge shouldn't break anything over there.