Add GoContext (#1140)
* Add GoContext
This collects up all dependancies that were on the toolchain, but shoudld be in
target configuration (rather than host) and migrates them to a common
go_context_data dependancy.
It then adds a GoContext object, that holds the toolchain, build mode, standard
library and all mode specific attributes. go_context(ctx) will build one of
these and prepare to use it.
This simplifies a lot of the logic in the rules, and helps enforce the isolation
of action methods from ctx attributes.
* Review feedback plus documentation
43 files changed
