Allow variable $FLATPAK_ID in modules.sources.path

[darkdragon-001]

Often files are named like the package. In order to only having to change the ID at a single place, it would be nice if variables like $FLATPAK_ID would be allowed everywhere in the modules section of the manifest, especially modules.sources.path and modules.build-options.env.

[hadess]

I wanted to use it in modules.config-opts so I could pass, for example, "-Dserver_name=${FLATPAK_ID}.dleyna-server instead of "-Dserver_name=org.gnome.Rhythmbox3.dleyna-server.