Notes: the AppendAncient plugin hook is broken by this commit.
This adds CaptureEnter() and CaptureExit() as no-ops for interface
compliance, but these capabilities should be added for plugin tracers
soon.
When the plugin loader itself had to know the types in the arguments
and return values of the plugin functions, it was very difficult to
avoid import loops, given that the types were often defined in the
same package that needed to invoke the plugins.
Under this model, the plugin loader has much less knowledge of the
plugins themselves, and within each package we define functions to
interact with the plugins.
