This removes a cyclical dependency when importing client/structs from
dependencies of the plugin_loader, specifically, drivers. Due to
client/config also depending on the plugin_loader.
It also better reflects the ownership of fingerprint structs, as they
are fairly internal to the fingerprint manager.
If someone wants to extend or reuse Cgroup detenction in the future they
can move `cgroup_linux.go` to `cgroup.go` and add the relevant build
tags.
Requested by: @dadgar
Change the logic from `!linux` to an empty build tag so that *if*
another platform picks up Cgroups support they can add themselves to the
necessary build tags for this fingerprinter and be on their way.
Because this technology isn't inherently Linux-specific and isn't
mutually exclusive of other resource isolation containers, resist the
urge to rename the Cgroup fingerprinter to something generic like the
ResourceContainerFingerprinter.
