go/internal/gcimporter: rename file to better reflect new meaning

Also: Removed unused deref function.

Follow-up on https://golang.org/cl/143023.

Updates #27999.

Change-Id: I088265f0e3ce52029679407f0b84b734191d4d3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/143024


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>