extract() does not correctly unnest paths in operators

[johngrimes]

[piotrszul]

@johngrimes as part of this I think I have change the way functions calls prefixes are combined - in general they are not combined with traversal prefixes as that would lead to these functions operating on $this in inside forEachOrNull which I do not think is correct (e.g. count() would always be 1 then).

An example is here:

au.csiro.pathling.extract.ExtractQueryTest#aggregations

    final ExtractRequest request = new ExtractRequestBuilder(subjectResource)
        .withColumn("id")
        .withColumn("name.family")
        .withColumn("name.first().family")
        .withColumn("name.count()")
        .build();

It converted to:

[DEBUG] a.c.p.extract.ExtractQueryExecutor - Unnested tree:
%resource
  id
    $this
  name.family
    $this
  name.first().family
    $this
  name.count()
    $this
[DEBUG] a.c.p.extract.ExtractQueryExecutor - Executing projection:
 group
  forEachOrNull: id
    columns
      one: $this as @pwed94
  forEachOrNull: name.family
    columns
      one: $this as @refdwb
  forEachOrNull: name.first().family
    columns
      one: $this as @2o59p6
  forEachOrNull: name.count()
    columns
      one: $this as @g7f9lf

and not something like:

%resource
  id
    $this
  name
     family
         $this
     first().family
         $this
    count()
        $this

(each leaf node is a column selection and each non-leaf node represent a forEachOrNull)

[piotrszul]

The operators are combines using their left argument as the prefix:

e.g: au.csiro.pathling.extract.ExtractQueryTest#combineWithUnequalCardinalities

    final ExtractRequest request = new ExtractRequestBuilder(subjectResource)
        .withColumn("id")
        .withColumn("name.given")
        .withColumn("name.family combine 'Smith'")
        .build();

is unnested as:

[DEBUG] a.c.p.extract.ExtractQueryExecutor - Unnested tree:
%resource
  id
    $this
  name
    given
      $this
    family.($this combine 'Smith')
      $this
[DEBUG] a.c.p.extract.ExtractQueryExecutor - Executing projection:
 group
  forEachOrNull: id
    columns
      one: $this as @d5v4gt
  forEachOrNull: name
    forEachOrNull: given
      columns
        one: $this as @sfhnrq
    forEachOrNull: family.($this combine 'Smith')
      columns
        one: $this as @7it6nj

family.($this combine 'Smith') may not be a parsable FHIR expression but it's a valid FhirPath so it executes correctly.