Track files installed by packages, allow query and use for removal

[AltGr]

Closes #502, #1215

[Drup]

So, this works for both .install files and the install field ? This is great \o/

[AltGr]

Yes, it includes all, so this is a bit redundant since it's an added mechanism, but that's not really a problem. Next, I intend to:

• make build dir unneeded by default for remove: (removing the need to download archives)
• add an uninstall-requires-build-dir flag
• write a repository-rewriting script to add the above flag where necessary or in doubt (using the infamous findlib-hack from opam, and removing it from opam proper)
• update the recommendations for the remove: field in the doc: just skip it if you just have to remove the files you added. Retaining the field for documentation is debatable, but since it's often incorrect, if we go that way CI should check that it indeed uninstalled properly (which is now possible).

Packages doing e.g. make uninstall will still need their sources though; maybe we could just suppress their remove: field in this case. Note that when there is no remove: field, the source is never needed, so removing most remove: fields rather than changing the download behaviour might be an alternative to the above.

A nice addition would be to not fail, but just skip the remove: step if the package archive can't be downloaded but we have a .changes file. Probably once I integrate the patch to do the removals as late as possible, and then include downloads in the actions processing graph rather than a first step.

[AltGr]

Where's Travis ?

[AltGr]

Filing #2571 seems to have wakened up Travis here too (well, it's the same commit). But in both, @github still displays the commits in the wrong order:
I locally have f10af22 before 7cf7b8d