Relative paths fix, Xcode generation at non-root path fix

[czechboy0]

Added support for properly creating relative paths even when the pivot isn't a full prefix of the path. Also fixed Xcode project generation at non-root path.

Properly finishes #188, together with #189.

[mxcl]

Can you describe the bug this fixes?

[czechboy0]

Sure. Previously the paths of source files in the generated projects were described relative to the package root (repo root), meaning they were all e.g. Sources/Build/build.swift. This worked fine assuming your generated Xcode project sits at the package root, as is the default.

With my change in #188, the project can be generated in a custom location, e.g. in ./XcodeProject/MyProject.xcodeproj, however then the path Sources/Build/build.swift doesn't point to an existing file, because there's no folder Sources in ./XcodeProject. So the path now needs to be ../Sources/Build/build.swift, relative path from the project to the source file.

This is why now instead of package.path we're using the Xcode project's path as the new root, because the path of every file in the Xcode project needs to be described relative to that project, not relative to the package root. That's why I'm passing in the Xcode project path and using that as the root.

The fixes in Utility's Path class are necessary to allow for path creation that contains optional .. components. Previously this wasn't supported, so if you tried to get a relative path from /A/B/C to /A/B/D, it didn't work. Now the right thing is generated: ../D.

Does that make sense? I tested generating Xcode projects at custom locations and the files or now properly referenced (they weren't in #188, that's my bad for not properly verifying the project worked).

[mxcl]

Great, thanks for the thorough explanation.

I was worried because I know we don't handle all the correct situations for relative paths, like for example /b/c relative to "../d", and I worry our code has bugs because of this.

[mxcl]

@swift-ci Please test

[czechboy0]

Hopefully not. I added tests for this new functionality of the relative path resolution and in separate PRs I'll start adding more tests for the utility classes in general, I feel like we should increase the test coverage there. 👍

[mxcl]

I feel like we should increase the test coverage there.

Oh, that would be wonderful.