cmd/go: fix build -o panic when import path pattern matches 0 pkgs

Fixes golang#8165.

After this change, the panic is replaced by a message:

        $ go build -o out ...doesntexist
        warning: "...doesntexist" matched no packages
        no packages to build

The motivation to return 1 exit error code is to allow -o flag
to be used to guarantee that the output binary is written to
when exit status is 0. If someone uses an import path pattern
to specify a single package and suddenly that matches no packages,
it's better to return exit code 1 instead of silently doing nothing.
This is consistent with the case when -o flag is given and multiple
packages are matched.
It's also somewhat consistent with the current behavior with the
panic, except that gave return code 2. But it's similar in
that it's also non-zero (indicating failure).
I've changed the language to be similar to output of go test
when an import path pattern matches no packages (it also has a return status of
1):

        $ go test ...doesntexist
        warning: "...doesntexist" matched no packages
        no packages to test

LGTM=adg
R=golang-codereviews, josharian, gobot, adg
CC=golang-codereviews
https://golang.org/cl/107140043

src/cmd/go/build.go

@@ -287,6 +287,8 @@ func runBuild(cmd *Command, args []string) {
 	if *buildO != "" {
 		if len(pkgs) > 1 {
 			fatalf("go build: cannot use -o with multiple packages")
+		} else if len(pkgs) == 0 {
+			fatalf("no packages to build")
 		}
 		p := pkgs[0]
 		p.target = "" // must build - not up to date