Use '_' instead of '$' in path names

The use of '$' in the path names for streams is a pain because, since
'$' is a special character in the shell, it makes it impossible to
directly copy and paste the paths. If we make this change, some builds
will be left with vestigial directories with $global and $build in them
until they run clean. It also would break any scripts that manually
delete these paths. That doesn't seem like a common use case, but it's
worth mentioning.

main/src/main/scala/sbt/internal/BuildStructure.scala

@@ -288,8 +288,8 @@ final class PartBuildUnit(
 object BuildStreams {
   type Streams = sbt.std.Streams[ScopedKey[_]]
 
-  final val GlobalPath = "$global"
-  final val BuildUnitPath = "$build"
+  final val GlobalPath = "_global"
+  final val BuildUnitPath = "_build"
   final val StreamsDirectory = "streams"
 
   def mkStreams(