Fix search for the Swift compiler on Windows

[egorzhdan]

Since the Path.isAbsolute implementation isn't valid for Windows paths, SwiftPM tries to find the Swift compiler in paths like this:

C:/Users/User/Documents/C:/Library/Developer/Toolchains/custom-development.xctoolchain/usr/bin/swiftc.exe

while the actual path is:

C:/Library/Developer/Toolchains/custom-development.xctoolchain/usr/bin/swiftc.exe

After this change SwiftPM can successfully find swiftc.exe on Windows, create a temporary directory for the artifacts and run the compiler (which currently results in an unrelated error).

[abertelrud]

First, thanks for the fixes! But, @aciidb0mb3r and @neonichu, the TSC in SwiftPM is just a vendored copy, right? These fixes should instead be made in that repository, right, and then the vendored copy should be updated? (or better yet, it should no longer be vendored)

[neonichu]

Yes, that's correct. I also remember there being a broader discussion about Windows support w.r.t. to file paths, but I don't know where that ended up.

[egorzhdan]

Ah, cool, thanks, I've misunderstood the readme (https://github.com/apple/swift-tools-support-core#development).
Created a new PR in the TSC repo: swiftlang/swift-tools-support-core#72

[egorzhdan]

Closing this PR since I've moved it to the TSC repo: swiftlang/swift-tools-support-core#72
Would greatly appreciate if someone could take a look at the changes 🙂