Improve output from rspec -v.

Before this change, this confusing thing could happen:

```
$ gem install rspec -v 3.5.0 > /dev/null; rspec -v
3.5.1
```

The user installed RSpec 3.5.0 but `rspec -v` prints 3.5.1.
This happened because 3.5.0 of the rspec gem depends on 3.5.x
of all the sub-gems, and 3.5.1 is the latest rspec-core.

With this change, it now prints something like:

```
$ rspec -v
RSpec 3.5
  - rspec-core 3.5.1
  - rspec-expectations 3.5.0
  - rspec-mocks 3.5.0
  - rspec-rails not installed
  - rspec-support 3.5.0
```

Author: myronmarston

Changelog.md

@@ -8,6 +8,8 @@ Enhancements:
 * Improve metadata filtering so that it can match against any object
   that implements `===` instead of treating regular expressions as
   special. (Myron Marston, #2294)
+* Improve `rspec -v` so that it prints out the versions of each part of
+  RSpec to prevent confusion. (Myron Marston, #2304)
 
 Bug Fixes:
 

lib/rspec/core/invocations.rb

@@ -49,7 +49,24 @@ def bisect_formatter_for(argument)
       # @private
       class PrintVersion
         def call(_options, _err, out)
-          out.puts RSpec::Core::Version::STRING
+          overall_version = RSpec::Core::Version::STRING
+          unless overall_version =~ /[a-zA-Z]+/
+            overall_version = overall_version.split('.').first(2).join('.')
+          end
+
+          out.puts "RSpec #{overall_version}"
+
+          [:Core, :Expectations, :Mocks, :Rails, :Support].each do |const_name|
+            lib_name = const_name.to_s.downcase
+            begin
+              require "rspec/#{lib_name}/version"
+            rescue LoadError
+              out.puts "  - rspec-#{lib_name} not installed"
+            else
+              out.puts "  - rspec-#{lib_name} #{RSpec.const_get(const_name)::Version::STRING}"
+            end
+          end
+
           0
         end
       end

spec/rspec/core/invocations_spec.rb

@@ -131,12 +131,46 @@ def run_invocation
     end
 
     describe Invocations::PrintVersion do
-      it "prints the version and returns a zero exit code" do
+      it "prints the major.minor version of RSpec as a whole" do
+        stub_const("RSpec::Core::Version::STRING", "9.18.23")
+        run_invocation
+        expect(out.string).to include("RSpec 9.18\n")
+      end
 
-        exit_code = run_invocation
+      it "prints off the whole version if it's a pre-release" do
+        stub_const("RSpec::Core::Version::STRING", "9.18.0-beta1")
+        run_invocation
+        expect(out.string).to include("RSpec 9.18.0-beta1\n")
+      end
 
-        expect(exit_code).to eq(0)
-        expect(out.string).to include("#{RSpec::Core::Version::STRING}\n")
+      it "prints off the version of each part of RSpec" do
+        [:Core, :Expectations, :Mocks, :Support].each_with_index do |const_name, index|
+          # validate that this is an existing const
+          expect(RSpec.const_get(const_name)::Version::STRING).to be_a String
+
+          stub_const("RSpec::#{const_name}::Version::STRING", "9.2.#{index}")
+        end
+
+        run_invocation
+
+        expect(out.string).to include(
+          "- rspec-core 9.2.0",
+          "- rspec-expectations 9.2.1",
+          "- rspec-mocks 9.2.2",
+          "- rspec-support 9.2.3",
+        )
+      end
+
+      it "indicates a part is not installed if it cannot be loaded" do
+        expect { require 'rspec/rails/version' }.to raise_error(LoadError)
+
+        run_invocation
+
+        expect(out.string).to include(" - rspec-rails not installed")
+      end
+
+      it "returns a zero exit code" do
+        expect(run_invocation).to eq 0
       end
     end