Earlier quoted context omitted.
Not just for a company, but for developer productivity in general, professional or not... Think of all the package managers that pull straight from Github repos as part of their installation processes..
Exactly ... we've got Ansible scripts that do that. One of the current Java rockstars (not my description), Adam Bien, recommends pulling the source code for all third-party dependencies to build them locally and put them in a local repository. I suppose you can push directly back upstream to avoid having an extra step.
If you're using Java most of the build tools use the Maven repositories, for which you can set up a local mirror too. The company i work for runs a Sonatype Nexus so we're not really affected by others' downtime unless we need stuff that we hadn't ever used before.