new-sdist fails with vcs packages

[fgaz]

$ cabal new-sdist some-vcs-package
cabal: The impossible happened: a local package isn't local

edit: here's the callstack:

CallStack (from HasCallStack):
  die', called at ./Distribution/Client/CmdSdist.hs:222:14 in main:Distribution.Client.CmdSdist

[quasicomputational]

We can probably fix this and #5586 at the same time.

[typedrat]

You shouldn't be able to sdist VCS packages at all, this is really just a dup of #5586 since that's what makes it possible for this to even be stated.

[23Skidoo]

Closing in favour of #5586, please reopen if disagree.