Allow for experiment tags to be non-exclusive

[matthewfeickert]

As we hopefully move to more software being shared (see also ACTS and co), it would be good if we had non-exclusive experiment tags.

Originally posted by @olantwin in #77

[kratsg]

As we hopefully move to more software being shared (see also ACTS and co), it would be good if we had non-exclusive experiment tags.

we can do this by flipping feedstocks.json to just list feedstocks and their associated tags instead. rather than tags->feedstocks.