Expand list of targets more efficiently

dcf has been using a tool called NYTProf to profile rbm: https://people.torproject.org/~dcf/graphs/nytprof-rbm-20200212-94fce31e/#subs_table

According to this, rbm has been spending a lot of time in the RBM::get_target function.

When the rbm configuration contains something like this:

targets:
  torbrowser-linux-x86_64:
    - linux-x86_64
    - linux

And the list of current targets is [ 'torbrowser-linux-x86_64' ], the RBM::get_targets function (which is calling RBM::get_target) is responsible for expanding the list of current targets to [ 'linux-x86_64', 'linux' ]. It is probably possible to re-implement this in a more efficient way.