Deal with special case where import is C.

Addresses #82.
diff --git a/cmd/guess_deps.go b/cmd/guess_deps.go
index f637c7c..53c8ab7 100644
--- a/cmd/guess_deps.go
+++ b/cmd/guess_deps.go
@@ -59,7 +59,7 @@
 		return err
 	}
 
-	if pkg.Goroot {
+	if pkg.Goroot || name == "C" {
 		return nil
 	}
 
diff --git a/cmd/tree.go b/cmd/tree.go
index 377aba1..6ec2043 100644
--- a/cmd/tree.go
+++ b/cmd/tree.go
@@ -121,6 +121,7 @@
 	ptypeVendor
 	ptypeGopath
 	ptypeGoroot
+	ptypeCgo
 )
 
 type pinfo struct {
@@ -165,6 +166,12 @@
 			return info
 		}
 	}
+
+	// Finally, if this is "C", we're dealing with cgo
+	if name == "C" {
+		info.PType = ptypeCgo
+	}
+
 	return info
 }