After sorting the step stack so that dependencies can be popped before
their dependants are popped, there is still a situation left to handle
correctly:
Example:
A depends on:
B
C
D depends on:
E
F
They will be ordered like this:
A B C D E F
If there are 6+ cores, then all of them will be evaluated at once,
incorrectly evaluating A and D before their dependencies.
Starting evaluation of F and then E is correct, but waiting until they
are done is not correct because it should start working on B and C as
well.
This commit solves the problem by computing dependants in the dependency
loop checking logic, and then having workers queue up their dependants
when they finish their own work.
And make it not do any installation, only objcopying. We already have
install steps for doing installation.
This commit also makes ObjCopyStep properly integrate with caching.
Usage of `catch unreachable` in build scripts is completely harmless
because build scripts are always run in Debug mode, however, it sets a
poor example for beginners to learn from.
