refactor(plugins): Improved and simplified verifier and plugin list

* Added proper executable check for current user
* Refactor plugin_list to make it more straightforward
* Seperate errors and warnings
* Don't return an error when no plugin is installed and `plugin list` is called
* Simplified tests
* Check assumption that a prefix is given in verify()

docs/cmd/kn_plugin.md

@@ -31,5 +31,5 @@ kn plugin [flags]
 ### SEE ALSO
 
 * [kn](kn.md)	 - Knative client
-* [kn plugin list](kn_plugin_list.md)	 - List all visible plugin executables
+* [kn plugin list](kn_plugin_list.md)	 - List plugins
 

docs/cmd/kn_plugin_list.md

@@ -1,16 +1,16 @@
 ## kn plugin list
 
-List all visible plugin executables
+List plugins
 
 ### Synopsis
 
-List all visible plugin executables.
+List all installed plugins.
 
-Available plugin files are those that are:
+Available plugins are those that are:
 - executable
-- begin with "kn-
-- anywhere on the path specified in Kn's config pluginDir variable, which:
-  * can be overridden with the --plugin-dir flag
+- begin with "kn-"
+- Kn's plugin directory ~/.kn/plugins
+- Anywhere in the execution $PATH (if lookupInPath config variable is enabled)
 
 ```
 kn plugin list [flags]
@@ -23,6 +23,7 @@ kn plugin list [flags]
       --lookup-plugins-in-path   look for kn plugins in $PATH
       --name-only                If true, display only the binary name of each plugin, rather than its full path
       --plugins-dir string       kn plugins directory (default "~/.kn/plugins")
+      --verbose                  verbose output
 ```
 
 ### Options inherited from parent commands

go.mod

@@ -25,7 +25,7 @@ require (
 	github.com/mitchellh/go-homedir v1.1.0
 	github.com/modern-go/reflect2 v1.0.1 // indirect
 	github.com/peterbourgon/diskv v2.0.1+incompatible // indirect
-	github.com/pkg/errors v0.8.1 // indirect
+	github.com/pkg/errors v0.8.1
 	github.com/spf13/cobra v0.0.3
 	github.com/spf13/pflag v1.0.3
 	github.com/spf13/viper v1.3.1

pkg/kn/commands/plugin/plugin_handler.go

@@ -62,7 +62,7 @@ func (h *DefaultPluginHandler) Lookup(name string) (string, bool) {
 		pluginPath := fmt.Sprintf("%s-%s", prefix, name)
 
 		// Try to find plugin in pluginsDir
-		pluginDir, err := ExpandPath(h.PluginsDir)
+		pluginDir, err := expandPath(h.PluginsDir)
 		if err != nil {
 			return "", false
 		}