syncpackage: downloads .dsc when it really isn't needed

[impact]

syncpackage saves the dsc file(s) to disk but doesn't need to

[test case]

ubuntu@lp1928946-h:~$ syncpackage --simulate --no-lp -r bionic ubuntu-dev-tools
The use of --no-lp is not recommended for uploads targeted at Ubuntu. The archive-admins discourage its use, except for fakesyncs.
Downloading ubuntu-dev-tools_0.181.dsc from deb.debian.org (0.002 MiB)
Using existing file ubuntu-dev-tools_0.181.dsc
Downloading ubuntu-dev-tools_0.164.dsc from archive.ubuntu.com (0.002 MiB)

check if the dsc file(s) are now in $PWD:

ubuntu@lp1928946-h:~$ ls -l *dsc
-rw-rw-r-- 1 ubuntu ubuntu 2356 Jun 2 15:40 ubuntu-dev-tools_0.164.dsc
-rw-rw-r-- 1 ubuntu ubuntu 2219 Jun 2 15:40 ubuntu-dev-tools_0.181.dsc

[regression potential]

any regression would likely cause incorrectly saved dsc files, or incorrect failure to save a dsc file to disk when using any of the various scripts included in the ubuntu-dev-tools pacakge, or other scripts that use the python3-ubuntutools package library

[scope]

this is needed only for h and later

this was introduced by commit fed562405d76ab85e2aab96ec0357457eebafcb2 which is included in 0.179, so this bug does not exist in g or earlier

[original description]

It seems that starting from a few versions, syncpackage downloads .dscs, stores them in ~, and leaves them there.

AFAIK there is no need to download anything to sync a package, as launchpad should be able to handle everything through its API, so I'd say the bug is threefold:

1) shouldn't download anything if not needed (when is it needed?!)
2) shouldn't store things in ~ at all, use a tempdir in TMPDIR instead
3) shouldn't leave temporary files around

Currently running version 0.181 under Debian unstable.