previously, for a directory on the classpath,
all files and directories under it would be checked
this caused unnecessary setting recompilation when
non-classfiles changed in the directory
One example was the sbt.plugins file, which doesn't affect
compilation, and caused projects with plugins to take longer
than necessary to start up
- support import clauses
- error display: source name, line numbers for each expression
- for 'eval', 'get', 'set', and .sbt files, use default imports and import from Plugins, Builds