make it so plugins can have their own package.json

closes CNVS-36896

test plan: 
check this out
check out g/108509 in gems/plugins/analytics
run `yarn`
you should see it install graphael for analytics

Change-Id: Iee6a2d8bb2ee46d1cddc596b972d447f9bb3b1d6
Reviewed-on: https://gerrit.instructure.com/111765
Tested-by: Jenkins
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>

Author: ryankshaw

script/gem_npm

@@ -7,14 +7,14 @@
 # developing one of these, go into the dir and yarn there
 export NODE_ENV=production
 
-for path in $(ls -1 gems/*/package.json); do
-  cd ${path:0:${#path}-13}
+for path in $(ls -1 gems/{,plugins/}*/package.json 2> /dev/null); do
+  pushd $(dirname $path) > /dev/null
   if hash yarn 2>/dev/null; then
     echo "running yarn $1 for $path"
     yarn $1
   else
     echo "npm is deprecated in canvas-lms. You should start using yarn instead. see https://yarnpkg.com. Running npm $1 for $path"
     npm $1
   fi
-  cd ../..
+  popd > /dev/null
 done