Fix missing project for included remote file including with rules:exists

What does this MR do and why?

This MR resolves the bug when an included remote file has an include with rules:exists. A bug occurs because the remote context does not have a project.

Note: This MR does not change behavior. Normally, for include:rules in remote included files, we ignore other rules (if), but raise an error in exists. So, with this MR, we start ignoring exists.

Related to #365382 (closed)

More explanation;

If the remote file has rules inside of their includes, we cannot process them because we don't have enough context to know file changes, variables, etc.

Currently, when you define;

# http://example.com/random-address/include.yml

include:
  - remote: http://example.com/random-address/builds.yml
    rules:
      - if: $CI_COMMIT_BRANCH == "master"

the rule is always evaluated as false and the file will not be included.

However, when you define the rule with exists;

# http://example.com/random-address/include.yml

include:
  - remote: http://example.com/random-address/builds.yml
    rules:
      - exists: [Dockerfile]

we get an error.

We have this error because in lib/gitlab/ci/build/rules/rule/clause/exists.rb;

def satisfied_by?(_pipeline, context)
  paths = worktree_paths(context)

  exact_matches?(paths) || pattern_matches?(paths)
end

private

def worktree_paths(context)
  return unless context.project

  if @top_level_only
    context.top_level_worktree_paths
  else
    context.all_worktree_paths
  end
end

def exact_matches?(paths)
  @exact_globs.any? { |glob| paths.bsearch { |path| glob <=> path } }
end

• we don't have context, so we don't have context.project
• worktree_paths returns nil
• and we try to call methods on paths, which is nil.

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