Don't include full paths in package metadata

Makefile

 PYODIDE_ROOT=$(abspath .)
 include Makefile.envs
 
-FILEPACKAGER=emsdk/emsdk/emscripten/tag-1.38.10/tools/file_packager.py
+FILEPACKAGER=$(PYODIDE_ROOT)/emsdk/emsdk/emscripten/tag-1.38.10/tools/file_packager.py
 
 CPYTHONROOT=cpython
 CPYTHONLIB=$(CPYTHONROOT)/installs/python-$(PYVERSION)/lib/python$(PYMINOR)
@@ -64,7 +64,10 @@ build/pyodide.asm.js: src/main.bc src/jsimport.bc src/jsproxy.bc src/js2python.b
 
 
 build/pyodide.asm.data: root/.built
-	python $(FILEPACKAGER) build/pyodide.asm.data --preload root/lib@lib --js-output=build/pyodide.asm.data.js --use-preload-plugins
+	( \
+		cd build; \
+		python $(FILEPACKAGER) pyodide.asm.data --preload ../root/lib@lib --js-output=pyodide.asm.data.js --use-preload-plugins \
+  )
 	uglifyjs build/pyodide.asm.data.js -o build/pyodide.asm.data.js
 
 
@@ -136,7 +139,10 @@ build/test.data: $(CPYTHONLIB)
 	  cd $(CPYTHONLIB)/test; \
 	  find -type d -name __pycache__ -prune -exec rm -rf {} \; \
 	)
-	python $(FILEPACKAGER) build/test.data --preload $(CPYTHONLIB)/test@/lib/python3.6/test --js-output=build/test.js --export-name=pyodide --exclude \*.wasm.pre --exclude __pycache__
+	( \
+		cd build; \
+		python $(FILEPACKAGER) test.data --preload ../$(CPYTHONLIB)/test@/lib/python3.6/test --js-output=test.js --export-name=pyodide --exclude \*.wasm.pre --exclude __pycache__ \
+  )
 	uglifyjs build/test.js -o build/test.js
 
 

tools/buildpkg.py

@@ -143,14 +143,15 @@ def package_files(buildpath, srcpath, pkg, args):
     subprocess.run([
         'python',
         Path(os.environ['EMSCRIPTEN']) / 'tools' / 'file_packager.py',
-        buildpath / (name + '.data'),
+        name + '.data',
         '--preload',
         '{}@/lib/python3.6/site-packages'.format(libdir),
-        '--js-output={}'.format(buildpath / (name + '.js')),
+        '--js-output={}'.format(name + '.js'),
         '--export-name=pyodide',
         '--exclude', '*.wasm.pre',
         '--exclude', '__pycache__',
-        '--use-preload-plugins'], check=True)
+        '--use-preload-plugins'],
+        cwd=buildpath, check=True)
     subprocess.run([
         'uglifyjs',
         buildpath / (name + '.js'),