cmd/go/internal/modcmd: assign module's path and version to fileJSON when modFile...

cmd/go/internal/modcmd: assign module's path and version to fileJSON when modFile's module statement exists

Fixes panic on nil pointer dereference error when assigning module's path and version pair to fileJSON.

Fixes #31623

Change-Id: I3f61122ba0676a1270d3ad98900af8c8e9c90935
Reviewed-on: https://go-review.googlesource.com/c/go/+/173397
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>