Improve intra-target parallelism

Improve intra-target parallelism

With cross compilation soon on the horizon, it may be required to build a
library for both the host and target architectures. These two copies can
certainly be built in parallel. Additionally, all binaries produced by a package
can also be built in parallel, but are currently forced to be built
sequentially.

This commit improves this parallelism by allowing each job to create more work
before the package is considered built. Only after all targets have been built
is the new fingerprint written.