cmd/cover: error out if a requested source file contains a newline

cmd/cover: error out if a requested source file contains a newline

cmd/cover uses '//line' directives to map instrumented source files
back to the original source file and line numbers.
Line directives have no way to escape newline characters, so cmd/cover
must not be used with source file paths that contain such characters.

Updates #60167.

Change-Id: I6dc039392d59fc3a5a6121ef6ca97b0ab0da5288
Reviewed-on: https://go-review.googlesource.com/c/go/+/501577
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>