Currently, a multi-build requires overriding a lot of helpers to for
silly reasons:

 * dh_auto_*   => To set build dir
 * dh_install* => To set sourcedir

This is a lot of repeating that we could do without.  Ideally, we
would be able to chain all of this, so dh_auto_install puts it where
dh_install* expects it.  I.e. we need to link the builds to specific
binaries.  This has other advantages (such as dh_auto_* would be able
to skip irrelevant builds when packages are excluded).

The API:
========

This is what the packagers need to know:

 * "X-DH-Buildlabel: foo" in a binary package.

 * dh_auto_X --buildlabel foo

 * dh_auto_install --buildlabel foo defaults to --destdir debian/tmp-foo
   (except for --buildlabel <default>, which defaults to debian/tmp
    for backwards compatibility)

