fix(remote): use launchpad credentials from legacy location
If launchpad credentials do not exist in the new location
($XDG_DATA_HOME/snapcraft/launchpad-credentials) but exist in the legacy
location ($XDG_DATA_HOME/snapcraft/provider/launchpad/credentials), then
emit a deprecation warning and copy the credentials to the new location.
This commit mimicks the behavior for core22<= builds where gpg and dirmngr
are installed at runtime for projects with package-repositories. The trigger
for this commit is the fact that the environment where Snapcraft builds in
Launchpad do _not_ have the 'dirmngr' package installed.
Unfortunately the strategy of bundling 'gpg' and 'dirmngr' as stage-packages
in Snapcraft's own snap didn't work because gpg has the expected path to the
'dirmngr' executable hardcoded (always trying to call '/usr/bin/dirmngr').
