extract import map generation logic into utility

[thescientist13]

Related Issue

resolves #537 (as a result of looking into #557 (comment))

Summary of Changes

1. Extract logic from plugin-node-modules into its own utility
2. Also resolved refactor duplicate hardcoded dependencies list in plugin-graphql to a single shared data structure #538

TODOs

1. extend to graphql plugin
2. extend to babel plugin - remove need for supplemental runtime babel packages for plugin-babel  #906
   • Is going to require more work including installing dependencies and updating specs (there were none for this case!). Should make a new issue for the 1.0 backlog tracker to account for installing those deps to the package, copying into the tests folder, stubbing out package.json. Wonder if I could use walker package ranger to target against specific deps and build up the import map to shim that way??? 🧐

Discussion

1. make a package?
   • pass a path or an object? object assumes process.cwd()?
   • what kind of API?

Details

1. Should be merged _after_ #557 , to account for hardcoded usages of _node_modules_? ```js // use .mjs version if it exists, for packages like redux if (!Array.isArray(entry) && fs.existsSync(`${process.cwd()}/node_modules/${packageJson.name}/${entry.replace('.js', '.mjs')}`)) { entry = entry.replace('.js', '.mjs'); } ```

[thescientist13]

Some refactoring ideas coming out of #832

• just pass initial path to walkModule and let it do the fs.readFile?
• can't updateImportMap append /node_modules/ hint string?