Support package-names with dashes in them (#232)

Support package-names with dashes in them

We were smart about aliasing if you have name-collisions, but not if
your package name is something that's not a valid identifier, like
`"path/to/my-package"`, which Go for better or worse allows. Now we
remove all the invalid characters (in practice mainly dashes, dots, and
leading digits).

Fixes #231.

Test plan: make check

internal/testutil/junk---fun.name/types.go

@@ -0,0 +1,51 @@
+package testutil
+
+import (
+	"context"
+	"time"
+
+	"github.com/Khan/genqlient/graphql"
+)
+
+type ID string
+
+type Pokemon struct {
+	Species string `json:"species"`
+	Level   int    `json:"level"`
+}
+
+func (p Pokemon) Battle(q Pokemon) bool {
+	return p.Level > q.Level
+}
+
+type MyContext interface {
+	context.Context
+
+	MyMethod()
+}
+
+func GetClientFromNowhere() (graphql.Client, error)                    { return nil, nil }
+func GetClientFromContext(ctx context.Context) (graphql.Client, error) { return nil, nil }
+func GetClientFromMyContext(ctx MyContext) (graphql.Client, error)     { return nil, nil }
+
+const dateFormat = "2006-01-02"
+
+func MarshalDate(t *time.Time) ([]byte, error) {
+	// nil should never happen but we might as well check.  zero-time does
+	// happen because omitempty doesn't consider it zero; we'd prefer to write
+	// null than "0001-01-01".
+	//
+	// (I mean, we're tests.  Who cares!  But we may as well try to match what
+	// prod code would want.)
+	if t == nil || t.IsZero() {
+		return []byte("null"), nil
+	}
+	return []byte(`"` + t.Format(dateFormat) + `"`), nil
+}
+
+func UnmarshalDate(b []byte, t *time.Time) error {
+	// (modified from time.Time.UnmarshalJSON)
+	var err error
+	*t, err = time.Parse(`"`+dateFormat+`"`, string(b))
+	return err
+}