Upgrade local folders to volumes if applicable

Helps when a user selects an internal or external drive from the
local folder chooser.

This way we also mount any internal drives selected (previously
we just treated them as always-mounted).

https://bugs.launchpad.net/deja-dup/+bug/1886730
3 jobs for master in 4 minutes and 27 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #850026
flatpak
flatpak

00:04:27

passed #850024
reuse_lint

00:00:21

passed #850025
vala_lint

00:00:20