`ci_hsc` is now a metapackage

I’ve just merged DM-21410, which completes the transition from ci_hsc to testdata_ci_hsc, ci_hsc_gen2, and ci_hsc_gen3 by transforming the old ci_hsc package into a metapackage that simply declares a EUPS dependency on ci_hsc_gen2 and ci_hsc_gen3.

ci_hsc will remain a Git LFS package so previous revisions can be checked out correctly, though this may change in the future; the master branch is no longer includes any LFS-managed files. The nightly ci_hsc build should naturally just build the metapackage (and hence both real ci_hsc_gen* packages) from now on.