Check for webpacker being installed by having config file

[justin808]

Previously checking for the bin stubs, but some frameworks, like React
on Rails, don't use the binstubs.

[justin808]

@dhh @gauravtiwari this is the final change required by React on Rails.
See this comment from @gauravtiwari

[gauravtiwari]

@justin808 It's already done here:

https://github.com/rails/webpacker/blob/master/lib/tasks/webpacker/verify_install.rake#L6

And the check_binstubs change was introduced here: #381

[justin808]

@gauravtiwari Please try what you just said, by deleting the binstubs. You will see what I'm staying. The task depends on task verify_install: [:check_node, :check_yarn, :check_binstubs] do
so check_binstubs throws.

[gauravtiwari]

@justin808 Yepp I know, thinking a way around this ;). The change introduced in this PR is already present in verify_install task.

[dhh]

I'm not sure I follow this. What's the point of using these tasks that depend on binstubs if they binstubs are not there? Why not just have an alternate set of tasks for React on Rails that doesn't have these preconditions?

[justin808]

@dhh is right. I was trying to customize the compiler task in webpacker.yml to allow a custom task that would use nps or yarn. We can close this for now. The idea was that for non-js developers, compile could be set to true, and the JS would just work. However, on a giant project, the cost of checking for ever web request is too much.