Ensure every variable reference is passed to lookup function

[pietern]

Changes

References to keys that themselves are also variable references were shortcircuited in the previous approach. This meant that certain fields were resolved even if the lookup function would have instructed to skip resolution.

To fix this we separate the memoization of resolved variable references from the memoization of lookups. Now, every variable reference is passed through the lookup function.

Tests

Before this change, the new test failed with:

=== RUN   TestResolveWithSkipEverything
    [...]/libs/dyn/dynvar/resolve_test.go:208: 
        	Error Trace:	[...]/libs/dyn/dynvar/resolve_test.go:208
        	Error:      	Not equal: 
        	            	expected: "${d} ${c} ${c} ${d}"
        	            	actual  : "${b} ${a} ${a} ${b}"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-${d} ${c} ${c} ${d}
        	            	+${b} ${a} ${a} ${b}
        	Test:       	TestResolveWithSkipEverything