topdown/eval: conflict errors in functions when output not captured (#3920)

With func1.rego as

    package test
    r(x) {
      data.test.q(x)
    }
    q(_) = true
    q(_) = false

Before:

    $ opa eval -fpretty -d func1.rego 'data.test.r(1)'
    true

After:

    $ opa eval -fpretty -d func1.rego 'data.test.r(1)'
    1 error occurred: func1.rego:12: eval_conflict_error: functions must not produce multiple outputs for same inputs

Fixes #3912.

Also adds a CHANGELOG item making the backwards-compat-breaking change
prominent, and sharing an example of how to fix it.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

test/cases/testdata/functionerrors/test-functionerrors-1012.yaml

@@ -15,3 +15,18 @@ cases:
   query: data.test1.r = x
   want_error: functions must not produce multiple outputs for same inputs
   want_error_code: eval_conflict_error
+- data:
+  modules:
+  - |
+    package test
+
+    f(_) = true
+    f(_) = false
+
+    r {
+      data.test.f(1)
+    }
+  note: functionerrors/function output conflict, used as boolean
+  query: data.test.r = x
+  want_error: functions must not produce multiple outputs for same inputs
+  want_error_code: eval_conflict_error