cmd/go/internal/get: relax pathOK check to allow any letter

This fixes a regression of #18660 with the new path checks.

Updates #29230

Change-Id: I2dd9adab999e7f810e0e746ad8b75ea9622f56e7
Reviewed-on: https://team-review.git.corp.google.com/c/370578


Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-on: https://go-review.googlesource.com/c/154104


Reviewed-by: Bryan C. Mills <bcmills@google.com>