Support dot-notation when retrieving facts in facter_impl

In Puppet 8, core providers are being confined using facts fetched using
'dot-notation'. We need to support this style of lookup in our stub
implementation.

For example in `lib/puppet/provider/service/init.rb`

```ruby
confine :true => begin
  os = Puppet.runtime[:facter].value(:operatingsystem).downcase
  # ...
```

was updated to

```ruby
confine :true => begin
  os = Puppet.runtime[:facter].value('os.name').downcase
  # ...
```

See
puppetlabs/puppet@82cef23 for Puppet 8 change.

Relates to #38

lib/rspec-puppet/facter_impl.rb

@@ -8,7 +8,11 @@ def initialize
     end
 
     def value(fact_name)
-      @facts[fact_name.to_s]
+      begin
+        @facts.dig(*fact_name.to_s.split('.'))
+      rescue TypeError
+        nil
+      end
     end
 
     def clear

spec/unit/facter_impl_spec.rb

@@ -10,6 +10,7 @@
       'int_fact' => 3,
       'true_fact' => true,
       'false_fact' => false,
+      'os' => { 'name' => 'my_os', 'release' => { 'major' => '42' } }
     }
   end
 
@@ -19,6 +20,7 @@
     facter_impl.add(:int_fact) { setcode { 3 } }
     facter_impl.add(:true_fact) { setcode { true } }
     facter_impl.add(:false_fact) { setcode { false } }
+    facter_impl.add(:os) { setcode { { 'name' => 'my_os', 'release' => { 'major' => '42'  }} } }
   end
 
   describe 'noop methods' do
@@ -49,6 +51,28 @@
     it 'retrieves a fact of type FalseClass' do
       expect(facter_impl.value(:false_fact)).to eq(false)
     end
+
+    context 'when using dot-notation' do
+      it 'retrieves a child fact using dot-notation' do
+        expect(facter_impl.value('os.name')).to eq('my_os')
+      end
+
+      it 'retrieves a hash child fact using dot-notation' do
+        expect(facter_impl.value('os.release')).to eq({ 'major' => '42' })
+      end
+
+      it 'retrieves a deeply nested child fact using dot-notation' do
+        expect(facter_impl.value('os.release.major')).to eq('42')
+      end
+
+      it 'returns nil if a child fact is missing' do
+        expect(facter_impl.value('os.release.unknown_subkey')).to eq(nil)
+      end
+
+      it 'returns nil if trying to lookup into a string' do
+        expect(facter_impl.value('os.name.foo')).to eq(nil)
+      end
+    end
   end
 
   describe '#to_hash' do