packaging: add Provides: python3-mercurial and Homepage to debian package

There are other packages that depend on python3-mercurial, like debian's
mercurial-git, so we should mark ourselves as providing it.

I compared the control file we generate to the one that the debian maintainers
generate, and noticed several differences:
- the Homepage bit. I included this, because why not
- a more robust Suggests list that includes a graphical merge tool
- a more robust Breaks list
- debian's Recommends openssh-client, we only Recommends ca-certificates
- a split into `mercurial` and `mercurial-common` (and possibly others?)
- a slightly different description

Differential Revision: https://phab.mercurial-scm.org/D9983

contrib/packaging/debian/control

@@ -25,7 +25,9 b' Recommends: ca-certificates'
 Suggests: wish
 Replaces: mercurial-common
 Breaks: mercurial-common
+Provides: python3-mercurial
 Architecture: any
+Homepage: https://www.mercurial-scm.org/
 Description: fast, easy to use, distributed revision control tool.
  Mercurial is a fast, lightweight Source Control Management system designed
  for efficient handling of very large distributed projects.