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 puppetlabs#38

lib/rspec-puppet/facter_impl.rb

@@ -8,7 +8,13 @@ def initialize
  end
 
  def value(fact_name)
- @facts[fact_name.to_s]
+ fact_name.to_s.split('.').reduce(@facts) do |sub_hash, key|
+ if sub_hash.is_a?(Hash) && sub_hash.key?(key)
+ sub_hash[key]
+ else
+ return nil
+ end
+ 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 +51,24 @@
  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
+ end
  end
 
  describe '#to_hash' do