Provide a way to get source for a snap in the store

[galgalesh]

@Dominic108

Of course, this is not enough, because we need the actual source code and this means knowing the specific build. If the source code was adapted by snap maintainers and does not correspond to any build, then it is not less important, because the VLC licence, the GNU license, says

Can you report this as an issue to VLC? Since they build the snap package, they are responsible to add the manifest. They can instruct snapcraft to add this manifest by setting the environment variable SNAPCRAFT_BUILD_INFO=1.