internal/imports: create listener map after clearing for new scan
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 <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools