Separate mutators and modules creation into a separate file in fsgen

filesystem_creator.go file is getting too large; this change separates
the filesystem_module definition and the fsgen package mutators and
modules creation logic.

Test: m nothing --no-skip-soong-tests
Change-Id: I832d4369984f3e45eb92278ea3a091f8c47f467e
diff --git a/fsgen/Android.bp b/fsgen/Android.bp
index e3cbdb3..690ad28 100644
--- a/fsgen/Android.bp
+++ b/fsgen/Android.bp
@@ -14,6 +14,8 @@
     ],
     srcs: [
         "filesystem_creator.go",
+        "fsgen_mutators.go",
+        "prebuilt_etc_modules_gen.go",
     ],
     testSrcs: [
         "filesystem_creator_test.go",