packer: use filepath for getting plugin basename

When Discovering plugins installed through the `Discover` function, we
use the base name of the plugin binary we discovered preliminarly, then
we match its name against a regex to extract the prefix for the plugin's
components.

Extracting the base path used to be done with `path.Base`, which while
working perfectly on UNIX systems, does not on Windows as it uses `\\`
as their path separator.

To circumvent this problem, we use the `filepath` package to extract the
base name of the plugin instead, making the discovery logic work again
on Windows.

packer/plugin.go

@@ -9,7 +9,6 @@ import (
 	"log"
 	"os"
 	"os/exec"
-	"path"
 	"path/filepath"
 	"regexp"
 	"runtime"
@@ -103,7 +102,7 @@ func (c *PluginConfig) Discover() error {
 	// We'll use that later to register the components for each plugin
 	pluginMap := map[string]string{}
 	for _, install := range installations {
-		pluginBasename := path.Base(install.BinaryPath)
+		pluginBasename := filepath.Base(install.BinaryPath)
 		matches := extractPluginBasename.FindStringSubmatch(pluginBasename)
 		if len(matches) != 2 {
 			log.Printf("[INFO] - plugin %q could not have its name matched, ignoring", pluginBasename)