Each bootstrapping stage ought to just depend on the previous stage, but
poorly-written compilers break this elegence. This provides an easy-enough
way to depend on the next stage: targetPackages. PLEASE DO NOT USE IT
UNLESS YOU MUST!
I'm hoping someday in a pleasant future I can revert this commit :)
This fixes the "sliding window" principle:
0. Run packages: build = native; host = foreign; target = foreign;
1. Build packages: build = native; host = native; target = foreign;
2. Vanilla packages: build = native; host = native; target = native;
3. Vanilla packages: build = native; host = native; target = native;
n+3. ...
Each stage's build dependencies are resolved against the previous stage,
and the "foreigns" are shifted accordingly. Vanilla packages alone are
built against themsevles, since there are no more "foreign"s to shift away.
Before, build packages' build dependencies were resolved against
themselves:
0. Run packages: build = native; host = foreign; target = foreign;
1. Build packages: build = native; host = native; target = foreign;
2. Build packages: build = native; host = native; target = foreign;
n+2. ...
This is wrong because that principle is violated by the target
platform staying foreign.
This will change the hashes of many build packages and run packages, but
that is OK. This is an unavoidable cost of fixing cross compiling.
The cross compilation docs have been updated to reflect this fix.
