Change install process to use system calls

Looks like the stderr stream _must_ be read from for some datastores
to install correctly. Switching to system() calls because that seems
to drain the streams without breaking.
All that's using it is "resilient_remove" which will calls it if it discovers a need to reinstall the package first. However if _install isn't good enough for general use we probably shouldn't be keeping it for a code path that will be rarely hit (if ever- most of this code came out of ancient tests for the original Python agent and much of it is over-cautious, which causes more trouble than it's worth.).