jiri: add pkg lookpath, and make gosh use it

Details:
- Moves lookpath functionality out of v.io/x/lib/envvar, to
  a new package named v.io/x/lib/lookpath
- Updates gosh to use this package, using the dirs from the
  Shell's PATH variable

Change-Id: I47bc80c527e54710d99c04dce0fbf8518f3a132c
MultiPart: 1/2
diff --git a/profiles/profilescmdline/manager_cmdline.go b/profiles/profilescmdline/manager_cmdline.go
index c329dee..110f9a3 100644
--- a/profiles/profilescmdline/manager_cmdline.go
+++ b/profiles/profilescmdline/manager_cmdline.go
@@ -278,7 +278,7 @@
 	if err == nil && fi.IsDir() {
 		env := cmdline.EnvFromOS()
 		env.Vars["PATH"] = jirix.Env()["PATH"]
-		return env.LookPathAll("jiri-profile", map[string]bool{})
+		return env.LookPathPrefix("jiri-profile", map[string]bool{})
 	}
 	return []string{}
 }