Add Rake/RequireOutsideTask cop

[corsonknowles]

What

Adds a new cop, Rake/RequireOutsideTask, that flags require/require_relative calls not inside a task block.

# bad
require 'foo'

task :foo do
  Foo.do_something
end

# bad - a namespace body is still evaluated on load
namespace :foo do
  require 'foo'

  task :bar do
    Foo.do_something
  end
end

# good
task :foo do
  require 'foo'
  Foo.do_something
end

# good
Rake::Task['db:seed'].enhance do
  require 'foo'
  Foo.do_something
end

Why

A rake file's top level — including the body of a namespace block — is evaluated every time the file is loaded (i.e. on every rake invocation), not only when the task runs. A require placed there is paid on every load. Moving it into the task (or a Rake::Task#enhance block) loads the dependency lazily, only when the task that needs it runs. This complements the existing Rake/MethodDefinitionInTask / Rake/ClassDefinitionInTask "the block scope is misleading" family.

Known tradeoff (why Enabled: pending, and the open question)

Some top-level requires must stay at load time because they feed task-defining DSLs — require "bundler/gem_tasks", require 'rspec/core/rake_task' (then RSpec::Core::RakeTask.new), require 'rubocop/rake_task' (then RuboCop::RakeTask.new).

This repo's own Rakefile has two such requires; We could mark them with inline # rubocop:disable Rake/RequireOutsideTask + a comment to demonstrate the escape hatch, but since Rakefile is the preferred place to do this over individual rake tasks, we exclude it from the rule by default.

Open questions for maintainers:

• Should the cop add significant new logic to exempt a require that is immediately followed by a task-defining call, rather than relying on inline disables for that case?
• Should the default Include be **/*.rake only (not Rakefile), since Rakefiles are the canonical place to require task libraries at the top? (currently, yes)

Origin:
extracted and generalized from a custom cop thst has run for over a year at Gusto.