Load plugins on demand

The following discussion from !81 (merged) should be addressed:

• @tristanvb started a discussion:

  Why would we want to ignore ImportError here ?

  I suspect this just snuck in to a commit accidentally, otherwise it should probably come with a comment explaining why we ignore that error.

The ImportError is required to suppress errors while loading the ostree source plugin if ostree is not installed; a better solution could be loading these plugins on-demand instead.