Add scoop prefix command (#2117)

Author: tresf

libexec/scoop-prefix.ps1

@@ -0,0 +1,25 @@
+# Usage: scoop prefix <app>
+# Summary: Returns the path to the specified app
+param($app)
+
+. "$psscriptroot\..\lib\core.ps1"
+. "$psscriptroot\..\lib\help.ps1"
+. "$psscriptroot\..\lib\manifest.ps1"
+. "$psscriptroot\..\lib\buckets.ps1"
+
+reset_aliases
+
+if(!$app) { my_usage; exit 1 }
+
+$app_path = versiondir $app 'current' $false
+if(!(Test-Path $app_path)) {
+    $app_path = versiondir $app 'current' $true
+}
+
+if(Test-Path $app_path) {
+    Write-Output $app_path
+} else {
+    abort "Could not find app path for '$app'."
+}
+
+exit 0