repart: Collect CopyFiles= lines into structs

repart: Collect CopyFiles= lines into structs

We currently convert the source:target pairs of the `CopyFiles=` lines
in `repart.d` files into a pairwise strv.  This works great if the only
thing that can be specified is a source and a target, but we're about to
add a flags field.

Let's start by making this a bit more explicit: we now turn each
`CopyFiles=` line into a `CopyFilesLine` struct.  We keep an array of
those in the `Partition` now, instead of the strv.

So far this is a whole lot of added complexity for nothing, but it's
necessary for the next step.

Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>