AvoidUsingPositionalParameters : rule is triggered only for the built-in functions

Steps to reproduce

function foo { param ( $a, $b, $c ) "boo" }

Invoke-ScriptAnalyzer -ScriptDefinition 'foo a b c'

Expected behavior

Rule AvoidUsingPositionalParameters is triggered.

Actual behavior

The rule is not triggered

Environment data

> $PSVersiontable  
Name                           Value                                                                                                                                                                                                                                                     
----                           -----                                                                                                                                                                                                                                                     
PSVersion                      6.0.1                                                                                                                                                                                                                                                     
PSEdition                      Core                                                                                                                                                                                                                                                      
GitCommitId                    v6.0.1                                                                                                                                                                                                                                                    
OS                             Linux 4.13.0-37-generic #42~16.04.1-Ubuntu SMP Wed Mar 7 16:03:28 UTC 2018                                                                                                                                                                                
Platform                       Unix                                                                                                                                                                                                                                                      
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                                                                                                                                                                                   
PSRemotingProtocolVersion      2.3                                                                                                                                                                                                                                                       
SerializationVersion           1.1.0.1                                                                                                                                                                                                                                                   
WSManStackVersion              3.0          

Start by reproducing the issue with the supplied PowerShell function and Invoke-ScriptAnalyzer command, then locate the AvoidUsingPositionalParameters rule in the repository. Trace why the rule handles built-in functions but not this user-defined function. Done means the supplied example triggers the rule and a regression test covers it.