message/pipeline: prepare for multi-stage extraction
message/pipeline: prepare for multi-stage extraction

most notably ssa and callgraph analysis

just restructuring now to keep diffs better later

Change-Id: I7e24053fe014430603a4daa282c87a97ad753433
Reviewed-on: https://go-review.googlesource.com/102715
Run-TryBot: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

golang/text