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          

首先，使用提供的 PowerShell 函数和 Invoke-ScriptAnalyzer 命令重现该问题，然后在仓库中找到 AvoidUsingPositionalParameters 规则。跟踪该规则为何能处理内置函数，却不能处理这个用户定义的函数。完成的标准是：提供的示例会触发该规则，并且有一个回归测试覆盖这一情况。