Add a lookup_path helper that returns full pack path based on name

[gauravtiwari]

Related to #231

# Production
irb(main):001:0> Webpacker::Manifest.lookup_path('admin/application.js')
=> #<Pathname:/Users/gaurav/webpacker-example-app/public/entries/admin/application-5fdeda182bb4fe694c49.js>
irb(main):002:0> Webpacker::Manifest.lookup_path('app.js')
=> #<Pathname:/Users/gaurav/webpacker-example-app/public/entries/app-ec0bee47eec8c98b0850.js>

# Development
irb(main):001:0> Webpacker::Manifest.lookup_path('app.js')
=> #<Pathname:/Users/gaurav/webpacker-example-app/public/entries/app.js>

@rmosolgo Is this good enough?

[rmosolgo]

Wow, yes, this looks great, thank you!

[ognjenio]

This has been changed and now it's:

Webpacker.manifest.lookup('calendar.js')

Ref: https://www.rubydoc.info/gems/webpacker/Webpacker%2FManifest:lookup