Fix PATH caching in CommandDiscovery code (#27809)

The method `GetLookupDirectoryPaths` always returns the cached instance of path collection. However, the returned collection gets mutated in `CommandPathSearch.ResolveCurrentDirectoryInLookupPaths` to resolve the relative paths such as `.\tools` based on the user's current working directory `$PWD`, so for example, `.\tools` gets replaced with `cwd-1\tools` in the cached instance. Then, when the user changes to a different working directory `cwd-2`, command discovery won't find the executable or ps1 script under `cwd-2/tools` as expected, but those executables under `cwd-1\tools` will always be discoverable no matter what the `$PWD` is. That behavior is incorrect.

This pull request improves the caching logic of `GetLookupDirectoryPaths` and makes it return a copy of the path collection, so the mutation happens to the returned copy, and the cached instance is kept intact. So, for every command search, the `CommandPathSearch` will resolve relative paths against the `$PWD` that the user is located at that time.

test/powershell/Modules/Microsoft.PowerShell.Core/Get-Command.Tests.ps1

@@ -275,3 +275,54 @@ Describe "Get-Command Tests" -Tags "CI" {
         $result | Should -Be $null
     }
 }
+
+Describe "Test relative path in PATH env var" -Tags "CI" {
+
+    BeforeAll {
+        $originalPath = $env:PATH
+        $originalLocation = Get-Location
+
+        $subDir1 = Join-Path $TestDrive -ChildPath "subdir1"
+        $subDir2 = Join-Path $TestDrive -ChildPath "subdir2"
+        $toolsUnderSubDir1 = Join-Path $subDir1 -ChildPath "tools"
+        $toolsUnderSubDir2 = Join-Path $subDir2 -ChildPath "tools"
+
+        $null = New-Item -Path $subDir1 -ItemType Directory -Force
+        $null = New-Item -Path $subDir2 -ItemType Directory -Force
+        $null = New-Item -Path $toolsUnderSubDir1 -ItemType Directory -Force
+        $null = New-Item -Path $toolsUnderSubDir2 -ItemType Directory -Force
+
+        $helloScript = Join-Path $toolsUnderSubDir1 -ChildPath "hello.ps1"
+        $byeScript = Join-Path $toolsUnderSubDir2 -ChildPath "bye.ps1"
+        $null = New-Item -Path $helloScript -ItemType File -Force
+        $null = New-Item -Path $byeScript -ItemType File -Force
+    }
+
+    AfterAll {
+        Set-Location $originalLocation
+        $env:PATH = $originalPath
+    }
+
+    It "Get-Command should resolve relative path in PATH env var based on user's current working directory" {
+        $dirSep = [System.IO.Path]::DirectorySeparatorChar
+        $pathSep = [System.IO.Path]::PathSeparator
+
+        Set-Location $subDir1
+        $env:PATH = ".${dirSep}tools${pathSep}$env:PATH"
+
+        $result = Get-Command "hello.ps1" -ErrorAction silentlycontinue
+        $result | Should -Not -BeNullOrEmpty -Because "CWD is $subDir1, so '.${dirSep}tools' in PATH should be resolved to '$toolsUnderSubDir1', which contains 'hello.ps1'"
+        $result.Path | Should -BeExactly $helloScript
+
+        $result = Get-Command "bye.ps1" -ErrorAction silentlycontinue
+        $result | Should -BeNullOrEmpty -Because "'bye.ps1' is not in '$toolsUnderSubDir1'"
+
+        Set-Location $subDir2
+        $result = Get-Command "hello.ps1" -ErrorAction silentlycontinue
+        $result | Should -BeNullOrEmpty -Because "CWD is $subDir2, so '.${dirSep}tools' in PATH should be resolved to '$toolsUnderSubDir2', which contains 'bye.ps1'"
+
+        $result = Get-Command "bye.ps1" -ErrorAction silentlycontinue
+        $result | Should -Not -BeNullOrEmpty
+        $result.Path | Should -BeExactly $byeScript
+    }
+}