Move wasm_backend.py to the matplotlib package

Makefile

@@ -187,7 +187,6 @@ root/.built: \
 		src/sitecustomize.py \
 		src/webbrowser.py \
 		src/pyodide.py \
-		src/wasm_backend.py \
 		remove_modules.txt
 	rm -rf root
 	mkdir -p root/lib
@@ -201,7 +200,6 @@ root/.built: \
 	cp src/_testcapi.py	root/lib/python$(PYMINOR)
 	cp src/pystone.py root/lib/python$(PYMINOR)
 	cp src/pyodide.py root/lib/python$(PYMINOR)/site-packages
-	cp src/wasm_backend.py root/lib/python$(PYMINOR)/site-packages
 	( \
 		cd root/lib/python$(PYMINOR); \
 		rm -fr `cat ../../../remove_modules.txt`; \

matplotlib/Makefile

@@ -85,6 +85,7 @@ $(BUILD)/__init__.py: $(HOSTBUILD)/lib.$(PLATFORMSLUG)/matplotlib/__init__.py
 	[ -d $(ROOT)/build ] || mkdir $(ROOT)/build
 	cp -r $(HOSTBUILD)/lib.$(PLATFORMSLUG)/matplotlib $(ROOT)/build && \
 	cp $(ROOT)/fontList.json $(BUILD)
+	cp $(ROOT)/src/wasm_backend.py $(BUILD)/backends
 	( \
 		cd $(BUILD); \
 		find . -name "*.so" -type f -delete; \

matplotlib/setup.cfg

 [rc_options]
-backend=module://wasm_backend
\ No newline at end of file
+backend=module://matplotlib.backends.wasm_backend