ninja: break unnecessary target dependencies
Previously, given two libraries, X and Y where X depends on Y, all object compilations of X would require the Y library to have been linked before being compiled. This is not necessary and can instead be loosened such that object compilations of X only depend on the order-only dependencies of Y to be completed. This is to ensure that generated sources, headers, custom commands, etc. are completed before X starts to compile its objects.
This should help build performance in projects with many libraries which cause a deep library dependency chain. Previously, a library at the bottom would not start compilation until after all other libraries completed, but now only its link step needs to wait and its compilation jobs can be run in parallel with other tasks.
Fixes #15555 (closed).