Plug hard-required by extra_applications

[IslandUsurper]

Existing Issue or Pull Request Verification

• Confirmed, no existing Issues or Pull Requests found related to my submission.

Package Version

2.17.3

Are you using the latest version?

• Yes, I verified the issue is still present in the latest version.

Steps to Reproduce

Create a library that does not depend on Plug, but does depend on stripity_stripe. Create a mix task to do anything at all, but with @requirements ["app.start"]. Try to run the task.

Expected Result

The mix task completes successfully.

Actual Result

10:29:23.746 [notice] Application decimal exited: :stopped
** (MatchError) no match of right hand side value: {:error, {:plug, {'no such file or directory', 'plug.app'}}}

[IslandUsurper]

Prior art: #701 and #798.
I believe the fix is to just remove :plug from the extra_applications in mix.exs. If Plug is present, it should be started automatically with other deps.

[srevenant]

Perhaps make a separate application for anything that requires :plug?
It is a little odd that :plug is a required dependency. That makes it hard for umbrella apps. Basically runs an assertion that people ONLY use this app within a phoenix app (which just isn't the case).