makedirs() function for transports

This seems reasonable, though there is still one question.

Which is whether it should fail if the target directory already exists.

os.makedirs() will raise an exception if the target exists, while 'mkdir -p' will not.

Most of our current code just silently swallows the exception that is naturally raised by the mkdir() call, so I would tend to implement it that way. What did you have in mind?

I'd tend to think it should be like mkdir -p. There might be cases
with locking where we want to see the exception. But I believe that
on some transports or filesystems we can't reliably detect that the
directory already exists (nfs?), so there's no point defining the
interface to require it.

I'd prefer 'mkdir -p' style behaviour. Most of the times I've ever wanted to do this sort of thing, I've just wanted to guarantee that the directory is there.

mwh points out this seems to be satisfied by Transport.create_prefix. Launchpad may want to make use of that (if you care) so I'll open a task there.

Launchpad is already using create_prefix.