cmd/go: ignore UTF8 BOM when reading source code

Fix the problem that UTF8 BOM can cause the parsing of import path and directives to fail.

Fixes #46198
Fixes #46290
Fixes #35726

Change-Id: I2d9995ee82b094bcfa5583f0cb4e8547cb973077
GitHub-Last-Rev: 98abf91377f155266fa60505c0c12beccad38eeb
GitHub-Pull-Request: golang/go#46643
Reviewed-on: https://go-review.googlesource.com/c/go/+/325990


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>