flatpak: Build primary module from upstream when not in the source tree

Currently when building from a flatpak manifest Builder expects it to be
in the project's source tree, so it builds it using the local project
checkout instead of fetching the upstream one.

But when there's a flatpak manifest not belonging to the project's
source tree (e.g. one of the flathub's app repository where there's only
the manifest), it still believes it makes sense to build locally.

But in this case it doesn't make sense at all, since it would be
expected to have Builder detect it's not in the project's source tree,
and proceed to build the primary module from upstream instead.

So this commit fixes this, by checking if the primary module matches
the project's root folder name, while ignoring case to detect it more
accurately, and if it's not detected as an in-tree manifest, then it
builds from upstream instead.
1 job for fix-flatpak-primary-module-thing in 33 minutes and 1 second (queued for 1 second)
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Status Job ID Name Coverage
  Test
passed #1130043
flatpak
flatpak

00:33:01