[Feature] Support Symbolic Links for Derived Data Directories

[granoff]

For users who may have moved their DerivedData directories to a removable drive, if a symlink to that relocated directory is placed inside ~/Library/Developer/Xcode, this change allows the tool to "just work" without the user needing to use the --custom-dervied-data-path command option (a nice option, but more complexity for scripts and such).

[FelixHerrmann]

Hey @granoff, apologize the delay here. I think symlinking Derived Data is not recommended nowadays but I'm fine getting this change in because it might still be utilized.

[granoff]

Thanks, Felix! My use case was actually to make this utility easier to use when you've moved, through Xcode settings the DerivedData directory. By placing a symlink to your new location in ~/Library... the tools works "by default" without having to redirect with a command line option. It's a convenience, sure. But for situations where this tool is used in a larger CI workflow, the nuance of the special directory handling is one less thing to worry about. :-) Thanks for merging!

[FelixHerrmann]

Interesting, I'll have this in mind for future development of this project; another information that is not bound to the project itself but to the environment where the project is build in.

BTW, the SPM plugin does not have this problem at all because we utilize the plugin work directory (within DerivedData) as the starting point to traverse up to the SourcePackages directory:
https://github.com/FelixHerrmann/swift-package-list/blob/master/Plugins/SwiftPackageListPlugin/SwiftPackageListPlugin.swift#L57-L67