When a non-relocatable application is packaged as a snap, its resources
will be visible under `${SNAP}`. We can discover these resources by
probing paths that are given to us and trying the same path under
`${SNAP}` if the probe fails.
Snap packages have desktop files namespaced by the snap name. This name
is available inside the snap environment as `${SNAP_NAME}`. Also probe
desktop files and try this form of mangled name if the referenced
desktop file is not found.
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