Also moves core.Key to label.Key, but leaves the implementations
behind for now.
After using for a while, the word Tag conveys slightly the wrong
concept, tagging implies the entire set of information, label maps
better to a single named piece of information.
A label is just a named key/value pair, it is not really tied to the
event package, separating it makes it much easier to understand the
public symbols of the event and core packages, and allows us to also
move the key implementations somewhere else, which otherwise dominate
the API.
Change-Id: I46275d531cec91e28af6ab1e74a2713505d52533
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229239
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
c81623a0cb