Source options

[yonaskolb]

This adds a new Source struct to replace the previous String model. This sets the stage to add various options to a targets sources by using a more expanded definition of a source.
Some possible we could build on top of this are:

• name - an optional override for the name of the file/directory
• compilerFlags
• excludes - maybe using a glob pattern
• group - could specify which group this sits in, defaults to the top level
• type - group or folder reference (defaults to group)
• buildPhase - overrides the guessing of what build phase files are for

These are now all valid ways to define a list of sources and are parsed into the same [Source] array:

sources: MySources
sources:
  - MySources
sources:
  - path: MySources