Update npm-package-arg@2.1.3

Bring in the latest version with fixed tests
Add realize-package-specifier@1.0.2
Use realize-package-specifier in cache/add
realize-package-specifier@1.1.0

Fix my misunderstanding of the difference between file: urls and local
paths.  The former are typically in package.json files and are relative to
the package root, where as local paths are typically specificied on the
command line and are relative to the current working directory.