Fix path handling in File-SD watcher to allow directory monitoring on Windows

Previously, `d.paths` were normalized to backslashes on Windows, even when if
the config file used Unix style.  The end result meant always watching `./`, so
changes for this config were always ignored:

    scrape_configs:
      - job_name: 'envmsc1'
        file_sd_configs:
        - files:
          - 'targets/envmsc1.d/*.yml'
          - 'targets/envmsc1.d/*.yaml'

Additionally, unlike the other platforms, no warning was emitted on startup
about not being able to install the watch if the directory didn't exist.  Now it
is logged.

Signed-off-by: Matt Harbison <mharbison@atto.com>

discovery/file/file.go

@@ -226,8 +226,8 @@ func (d *Discovery) watchFiles() {
 		panic("no watcher configured")
 	}
 	for _, p := range d.paths {
-		if idx := strings.LastIndex(p, "/"); idx > -1 {
-			p = p[:idx]
+		if dir, _ := filepath.Split(p); dir != "" {
+			p = dir
 		} else {
 			p = "./"
 		}