* Make it loose coupling between RubyGems and RDoc

\### Problems

There are following problems because of tight coupling between RubyGems and RDoc.

1. If there are braking changes in RDoc, RubyGems is also broken.
2. When we maintain RDoc, we have to change RubyGems.

The reason why they are happened is that RubyGems creates documents about a gem with installing it.

Note that RubyGems uses functions of RDoc to create documents.
Specifically,

- Creating documents is executed by `rubygems/lib/rubygems/rdoc.rb`.
- `::RDoc::RubygemsHook` which is defined by RDoc is called by the file.

\### Solution

RubyGems has the plugin system.

If a gem includes `rubygems_plugin.rb`, RubyGems loads it.
RubyGems executes a process defined in it while installing gems, uninstalling gems or other events.

We can use the system to solve the problems.

The root cause is RubyGems directly references the class of RDoc.

We can remove the root cause by making RDoc RubyGems plugin.

Alternatively `rubygems_plugin.rb` creates documents about gems.

\### FAQ

Q1. Do we need to change codes of RubyGems?

A.

No, we don't.

This change keeps compatibility of API used from RubyGems.

Q2. Is it better to delete existing codes related to RDoc in RubyGems?

No, it isn't.

If we change codes of RubyGems,
we can't keep a compatibility.

Example:

If we delete codes that uses `RDoc::RubygemsHook` in `rubygems/lib/rubygems/rdoc.rb`,
documentations are not created with old RDoc.

Q3. When can we delete `rubygems/lib/rubygems/rdoc.rb`?

A.

We can delete it when all users use RDoc including `rubygems_plugin`.

Next ruby version is 3.4.
If it includes the RDoc including `rubygems_plugin`,
we can delete `rubygems/lib/rubygems/rdoc.rb` after ruby 3.3 is EOL.

Q4. Is it a breaking change that Rubygems creates documents with
rubygems_plugin not RDoc::RubygemsHook?

A.

No, it isn't.

If we simply implement this approach,
we move the implementation from `rdoc/lib/rdoc/rubygems_hook.rb` to
`rubygems_plugin.rb`.

This way can be breaking change.

It seems to be fine that we just need to delete `rdoc/rubygems_hook.rb` but it doesn't work.
It generates multiple documents.

`rubygems/lib/rubygems/rdoc.rb` has the following code.

```
begin
  require "rdoc/rubygems_hook"
  # ...
rescue LoadError
end
```

This code ignores RDoc related processes when `rdoc/rubygems_hook` can't be required.
But, this 'require' is not failed.

This is because Ruby installs Rdoc as a default gem.

So, Rdoc installed as a default gem generates documents and one installed as a normal gem does it too.

If you think that this behavior is accectable,
we can just delete `rdoc/rubygems_hook.rb`.

What do you think about this approach?

In this change, we take another approach to solve the problem that creates multiple documents.

If `Gem.done_installing(&Gem::RDoc.method(:generation_hook))` in `rubygems/rdoc.rb` doesn't create documents,
we can solve the problem.

We have some options.

* We change `rubygems/rdoc.rb` and then don't execute `Gem.done_installing`.
  (This is a change for RubyGems.)
* We change `rdoc/rubygems_hook.rb` and then make `generation_hook` a no-op method.
  (This is a change for RDoc.)

We choose the latter to avoid changing for RubyGems.

\### Test

\#### Preparation

Install Rdoc which including our changes by executing `rake install`.

❯ rake install

We confirmed that Rdoc which including our changes was installed.

❯ gem list | grep rdoc
rdoc (6.6.0, default: 6.4.0)

\#### Check point

We tested to check compatibility.

How to chack the compatibility?

We tested creating same documents by our RDoc and old RDoc with latest RubyGems.

We used following versions to test.

```
❯ ruby -v
ruby 3.1.0p0 (2021-12-25 revision fb4df44) [arm64-darwin22]

❯ gem list | grep rdoc
rdoc (default: 6.4.0)

❯ ruby -I rubygems/lib rubygems/exe/gem --version
3.5.14
```

Here is a result of test with old RDoc.
We can see that the document is created correctlly with `Parsing...` and `Done installing...`.

```
❯ ruby -I rubygems/lib rubygems/exe/gem install pkg-config
Successfully installed pkg-config-1.5.6
Parsing documentation for pkg-config-1.5.6
Done installing documentation for pkg-config after 0 seconds
1 gem installed
```

Here is a result of test with our RDoc.
We can see that the document is created correctlly with `Parsing...` and `Done installing...`.

```
❯ ruby -I rubygems/lib rubygems/exe/gem install pkg-config
Successfully installed pkg-config-1.5.6
Parsing documentation for pkg-config-1.5.6
Done installing documentation for pkg-config after 0 seconds
1 gem installed
```

As you can see we got the same results, our RDoc keeps compatibility.

* rename a test file

* Revert "rename a test file"

This reverts commit 70a144bf3fb8f2cc653972e858b5fed3747765d7.

* revert a test class name

* exclude `TestRDocRubyGemsHook` at job of ruby-core

* When `rubygems_plugin.rb` is not found, `test_rdoc_rubygems_hook.rb` is skipped.

* remove unnecessary whitespace

* add comment

* Add support for the case that RDoc is installed as a default gem

* Fix problems

Co-authored-by: mterada1228 <49284339+mterada1228@users.noreply.github.com>

* Simplify

* removed unused blank lines and revert test

* for rerun tests

* add comment for rubygems_plugin.rb

---------

Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>