Give a better error if package_bindings is a file (#221)

The entries under the new `package_bindings` field should be packages,
but it's an easy mistake to put a file path instead (most of the other
fields in `genqlient.yaml` are files). Due to some bizzare behavior from
`go/packages` (described in #220), if you do that you get weird broken
code that gives you no clue what is wrong. Instead, let's guess if what
you gave us looks like a filename, and report a nice error if so.

Test plan: crossed fingers

generate/config.go

@@ -6,6 +6,7 @@ import (
 	"go/token"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"golang.org/x/tools/go/packages"
 	"gopkg.in/yaml.v2"
@@ -114,6 +115,14 @@ func (c *Config) ValidateAndFillDefaults(baseDir string) error {
 
 	if len(c.PackageBindings) > 0 {
 		for _, binding := range c.PackageBindings {
+			if strings.HasSuffix(binding.Package, ".go") {
+				// total heuristic -- but this is an easy mistake to make and
+				// results in rather bizarre behavior from go/packages.
+				return errorf(nil,
+					"package %v looks like a file, but should be a package-name",
+					binding.Package)
+			}
+
 			mode := packages.NeedImports | packages.NeedTypes | packages.NeedTypesSizes
 			pkgs, err := packages.Load(&packages.Config{
 				Mode: mode,