split upstream and our native into different source packages

split upstream and our native into different source packages

We should have never added a upstream repackage and some custom tools
in the same source package, let alone binary package. Finally resolve
that tech debt and do a split.

This commit does a bit more than I'd like at once, but it's really not
trivial to make a clean split while keeping all sanely buildable in
multiple commits.

What roughly happens is:
- two complete separate source packages
  - one for xterm.js, we keep this purely to the xterm.js and our
    local additions and modifications that are relevant directly to
    xterm.js itself
  - one for termproxy, where only the rust-based tool is shipped.
    This also reduces churn, as the termproxy doesn't sees lots of
    updates, so no point in building it on every xterm.js update.
  - drop debcargo, which while great for library source code packages
    for build dependencies, is just a PITA for binary packages
    shipping actual compiled rust code, it adds almost nothing there
    but provides lots of special handling to get the correct result.
  - rename `termproxy` to `proxmox-termproxy` but keep a backward
    compat link for now, termproxy is way to general a name.
    Maybe we should also move this away from /bin in general, into the
    /lib/<arch-qualifier>/proxmox/ directory like we do for
    proxmox-backup.
  - clean ups all over the place

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>