on_macos/on_linux block: improve rubocop message

[SeekingMeaning]

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• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
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Related: #9470 (comment)

Changes:

• Include the name and line number of the method that isn't allowed
• Print the list of all allowed methods

Before:

% brew style util-linux
Library/Taps/homebrew/homebrew-core/Formula/util-linux.rb:19:3: C: on_macos can only include depends_on, patch and resource nodes.
  on_macos do ...
  ^^^^^^^^^^^

After:

% brew style util-linux
Library/Taps/homebrew/homebrew-core/Formula/util-linux.rb:20:5: C: on_macos cannot include keg_only.
Only these are allowed: depends_on, patch, resource, deprecate!, disable!
    keg_only "macOS provides the uuid.h header"
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[BrewTestBot]

Review period will end on 2020-12-10 at 17:05:22 UTC.

[SeekingMeaning]

components_order_spec expects the old message, and updating it to check for multiline messages was harder than I thought as the parser doesn't seem to support multiline messages

Here are the tests in question: https://github.com/Homebrew/brew/blob/master/Library/Homebrew/test/rubocops/components_order_spec.rb#L388-L412

^^^^^^^^^^^ `on_macos` can only include `depends_on`, `patch` and `resource` nodes.

Our best available options, I think:

1. Shorten the message to only this:

`on_macos` cannot include `keg_only`.

2. Or shorten the message to only this:

`on_macos` can only include `depends_on`, `patch`, `resource`, `deprecate!`, `disable!`

3. Or put the entire message on one line:

`on_macos` cannot include `keg_only`. Only these are allowed: `depends_on`, `patch`, `resource`, `deprecate!`, `disable!`

[reitermarkus]

I found you can abbreviate the message with [...], but it still cannot be multiline because the span will not match on the second line.

[reitermarkus]

Another bug I found: If the block only contains one node which isn't allowed, it will not catch it since I guess the whole cop only applies if there are multiple components in the block.

[reitermarkus]

I'm not quite sure why this is even part of ComponentsOrder and not a separate cop.

[SeekingMeaning]

Another bug I found: If the block only contains one node which isn't allowed, it will not catch it since I guess the whole cop only applies if there are multiple components in the block.

#9470 fixes this

[BrewTestBot]

Review period ended.