Should allow specifying ordering of copy files phases

[brentleyjones]

From Xcode 10 release notes:

While a target is performing post-compilation build steps, such as linking and code signing, Xcode’s new build system will begin compiling sources from targets that depend on that target, to improve build speed by using more of your available processor cores. This compilation will not start until any “Run Script” phases from the target have completed. (38772619)

We should allow specifying of the ordering of copy file phases. Scripts will block the compilation of a dependent target, while copy files (probably) doesn't. Being able to determine the order of these can improve build times, but also impact correctness of a build. The copy files phase might have to come before a post-build run script or even before Compile Sources, in order to allow dependent targets to compile correctly.

[brentleyjones]

Without a change you can get build errors like this:

error: Cycle in dependencies between targets 'Harmony' and 'Routing'; building could produce unreliable results. This usually can be resolved by moving the target's Headers build phase before Compile Sources.
Cycle path: Harmony → Routing → Harmony
Cycle details:
→ Target 'Harmony' has target dependency on Target 'Routing'
→ Target 'Routing': Libtool /Users/z002wrr/Library/Developer/Xcode/DerivedData/Harmony-dobqplhmutrloybkgkxvysfcinjt/Build/Products/Debug-iphonesimulator/libRouting.a normal x86_64
○ Target 'Routing' has compile command for Swift source files
○ Target 'Routing' has copy command from '/Users/z002wrr/Development/Target/Harmony-iOS/Routing/Sources/module.modulemap' to '/Users/z002wrr/Library/Developer/Xcode/DerivedData/Harmony-dobqplhmutrloybkgkxvysfcinjt/Build/Products/Debug-iphonesimulator/include/Routing/module.modulemap'

Because something like a modulemap is needed for compilation of targets depending on this target.