dawn_node: Move cmd/idlgen under a src directory

Creates a more idiomatic golang source tree.

Change-Id: Ifdb701e5aa2f391ca23d957bee59412a8cf7139d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/65420
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_node/CMakeLists.txt

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 set(GEN_DIR         "${CMAKE_CURRENT_BINARY_DIR}/gen")
-set(IDLGEN_TOOL_DIR "${CMAKE_CURRENT_SOURCE_DIR}/tools/cmd/idlgen")
+set(IDLGEN_TOOL_DIR "${CMAKE_CURRENT_SOURCE_DIR}/tools/src/cmd/idlgen")
 
 # idlgen() is a function that uses the tools/cmd/idlgen/main.go tool to generate
 # code from an IDL file and template.