@Override
            public void freeze() {
                allComponents().forEach(Target::freeze);
                super.freeze();
            }
Resolves (source, provider) component specifications to a search chain invocation spec.
The provider component specification is given by the entry in the queryMap with key
'source.&lt;source-name&gt;.provider'.

<p>
The diagram shows the relationship between source, provider and the result:
(source is used to select row, provider is used to select column.)
Provider id = null is used for regular search chains.
</p>

<pre>
                  Provider id
                null
               |----+---+---+---|
               | o  |   |   |   |
               |----+---+---+---|
Source id      |    | o | o |   |
               |----+---+---+---|
               |    |   |   | o |
               |----+---+---+---|

                   o: SearchChainInvocationSpec
</pre>

@author Tony Vaagenes
container-search/src/main/java/com/yahoo/search/federation/sourceref/SearchChainResolver.java