[WIP] Add a Dep closure script.

[abadger]

This checks whether a set of collections contain all of their own
depoendencies. The intention is that this can be run on the ansible
package to tell if there are any missing collections or conflicting
collection versions in the package.

Fixes #268

TODO

• Make the check a subcommand of antsibull-lint so people can run it on their arbitrary set of collections.
• Extract the checking code so that we can run it as part of antsibull-build as well
• As part of antsibull-build, generate warnings if there are dependency errors
• Enhance the nightly build to notify if the warnings are being triggered

[abadger]

If someone else wants to take this over, that would be fine. I just had some free time to get started on it.

I've asked @felixfontein for an opnion on whether we should integrate this into antsibull-lint or leave it as its own script.

[felixfontein]

I rebased this commit and converted it to an antsibull-build subcommand in #416.