Add more paths helper methods

Add PathsForOutput to convert multiple strings into WritablePaths.

Add OutputPath.InSameDir to build a new OutputPath pointing to a
file in the same directory as an existing OutputPath.

Add WritablePathForTesting and WritablePathsForTesting that mirror
PathForTesting and PathsForTesting but return WritablePaths.

Add PathContextForTesting to return a minimal PathContext
implementation.

Test: paths_test.go
Change-Id: I9708eb164b273514a96dae0a260ef9a963fb9bcf