Clarify usages of shared source dir in Build.scala, replace literals with val.

Users may want to change the name of the "scala" shared source directory.  This patch doesn't change any of the directory names, but just makes it clear how to change the shared source directory ("scala") by replacing two literal "scala" strings with usage of a new val, sharedSrcDir.  Also function "addSrcSetting" was renamed to "addSharedSrcSetting" for clarity.