internal/imports: create listener map after clearing for new scan

The module resolver needs to recreate the listener map if it's cleared
for a new scan.

Change-Id: If5e945d4f2059f2a79aef3129f963a2c50e90229
Reviewed-on: https://go-review.googlesource.com/c/tools/+/214278
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/imports/mod.go

@@ -175,7 +175,8 @@ func (r *ModuleResolver) ClearForNewScan() {
 	<-r.scanSema
 	r.scannedRoots = map[gopathwalk.Root]bool{}
 	r.otherCache = &dirInfoCache{
-		dirs: map[string]*directoryPackageInfo{},
+		dirs:      map[string]*directoryPackageInfo{},
+		listeners: map[*int]cacheListener{},
 	}
 	r.scanSema <- struct{}{}
 }