Fix generating source links in api documentation.  This involved:
  * Removing macro classes from doc compile classpath
  * Copying macro sources in docsrc, with corresponding directory structure
  * Adding -Ymacro-no-expand to doc generation scalac options.
  * Refactoring genDocSources to allow multiple srcDirs
  * Changing docSourceUrl to top-level of GitHub tree
  * Changing genscalactic source generation to use sourceDirectory instead of sourceManaged (so that matches on src/ prefix)