Move android/soong/fs to android/soong/finder/fs

The fs package is specific to finder, move it inside finder.

Bug: 70897635
Test: m checkbuild
Change-Id: Ie705f064a832141702a8e87fd59ed75c01018504
diff --git a/ui/build/finder.go b/ui/build/finder.go
index a0f5d08..3bd6d87 100644
--- a/ui/build/finder.go
+++ b/ui/build/finder.go
@@ -16,7 +16,7 @@
 
 import (
 	"android/soong/finder"
-	"android/soong/fs"
+	"android/soong/finder/fs"
 	"android/soong/ui/logger"
 	"bytes"
 	"io/ioutil"