feat: make lake clean clean all packages

Fixes leanprover/lake#155

Lake/CLI/Main.lean

@@ -296,7 +296,7 @@ protected def printPaths : CliM PUnit := do
 protected def clean : CliM PUnit := do
   processOptions lakeOption
   let config ← mkLoadConfig (← getThe LakeOptions)
-  noArgsRem do (← loadWorkspace config).root.clean
+  noArgsRem do (← loadWorkspace config).clean
 
 protected def script : CliM PUnit := do
   if let some cmd ← takeArg? then

Lake/Config/Workspace.lean

@@ -178,3 +178,9 @@ def augmentedEnvVars (self : Workspace) : Array (String × Option String) :=
     ("LEAN_SRC_PATH", some self.augmentedLeanSrcPath.toString),
     (sharedLibPathEnvVar, some self.augmentedSharedLibPath.toString)
   ]
+
+/-- Remove all packages' build outputs (i.e., delete their build directories). -/
+def clean (self : Workspace) : IO Unit := do
+  for (_, pkg) in self.packageMap do
+    pkg.clean
+  self.root.clean