Ensure that there is a trailing '/' for mapSourceURI settings.
Ensure that there is a trailing '/' for mapSourceURI settings.

If the base directory does not exist yet at the time the setting
is constructed, the URI did not contain a trailing '/'. That could
cause spurious full recompilation during the *second* run after a
'clean'. We now ensure that there is always a trailing '/'.

scala-js/scala-js,sjrd/scala-js,sjrd/scala-js,gzm0/scala-js,scala-js/scala-js,sjrd/scala-js,gzm0/scala-js,scala-js/scala-js