Update the gtest branch to main, fix rpg_baselines refs

[nacorti]

Howdy y'all! Fixing a couple of issues I ran into while getting this thing running on a GCP Ubuntu 18.04 instance.

• googletest has updated their main branch from master, see this issue

• the import from rpg_baselines.common seems to fail without the extra packages added to the cmake file, see this issue

If there's any other docs or PR etiquette I'm missing, please do let me know and I'll fix ASAP.