Every time a gem is not found in the Compact Index API, RubyGems will
fallback to the full index, which is very slow. This is unnecessary
because both indexes should be providing the same gems, so if a gem
can't be found in the Compact Index API, it won't be found in the full
index.

We _do_ want a fallback to the full index, whenever the Compact Index
API is not implemented. To detect that, we check that the API responds
to the "/versions" endpoint, just like Bundler does.

Before:

```
$ time gem install fooasdsfafs
ERROR:  Could not find a valid gem 'fooasdsfafs' (>= 0) in any repository
gem  20,77s user 0,59s system 96% cpu 22,017 total
```

After:

```
$ time gem install fooasdsfafs
ERROR:  Could not find a valid gem 'fooasdsfafs' (>= 0) in any repository
gem  5,02s user 0,09s system 91% cpu 5,568 total
```

https://github.com/rubygems/rubygems/commit/c0d6b9eea7

It is not an empty gzipped data, a gzipped empty dump data.

Be careful when writing binary data on Windows.

```
$ ruby -e 's = Gem::Util.gzip("\x04\x08[\x05".b); p s.index("\n"); puts IO::Buffer.for(s).hexdump'
6
0x00000000  1f 8b 08 00 6c 3d 0a 66 00 03 63 e1 88 66 05 00 ....l=.f..c..f..
0x00000010  e3 69 10 89 04 00 00 00                         .i......
```

https://github.com/rubygems/rubygems/commit/f25013bcc0

https://github.com/rubygems/rubygems/commit/34df962cf4

https://github.com/rubygems/rubygems/commit/99d91c9ed2

So generate_index can be implemented with dependencies, such as the compact index

Took this approach from feedback in https://github.com/rubygems/rubygems/pull/6853

Running `gem generate_index` by default will use an installed rubygems-generate_index, or install and then use the command from the gem

Apply suggestions from code review

https://github.com/rubygems/rubygems/commit/fc1cb9bc9e

Co-authored-by: Hiroshi SHIBATA <hsbt@ruby-lang.org>

https://github.com/rubygems/rubygems/commit/4eaac27107