imports: make ScanDir ignore go files start with dot

Adding "." Prefix Check for go files.

Fixes #42047
Change-Id: Ifc42bf562f52fdd304f9828b06fc57888fcd8049
Reviewed-on: https://go-review.googlesource.com/c/go/+/264078


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>