Adds support for mojo apps to live in their own directory

mojo/shell/BUILD.gn

Index: mojo/shell/BUILD.gn
diff --git a/mojo/shell/BUILD.gn b/mojo/shell/BUILD.gn
index 38b8639f7807aa1a93b8bf34dc6b96c46ac767b9..9f2d90d4a626dd9e20306464e5b82abc2e160670 100644
--- a/mojo/shell/BUILD.gn
+++ b/mojo/shell/BUILD.gn
@@ -95,8 +95,6 @@ source_set("lib") {
     "child_process_host.h",
     "context.cc",
     "context.h",
-    "filename_util.cc",
-    "filename_util.h",
     "init.cc",
     "init.h",
     "out_of_process_native_runner.cc",
@@ -128,7 +126,7 @@ source_set("lib") {
   ]
 
   public_deps = [
-    ":switches",
+    ":common",
   ]
 
   if (is_android) {
@@ -187,8 +185,10 @@ source_set("native_application_support") {
   check_includes = false
 }
 
-source_set("switches") {
+source_set("common") {
   sources = [
+    "filename_util.cc",
+    "filename_util.h",
     "switches.cc",
     "switches.h",
   ]