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

generate/generate_test.go

@@ -171,6 +171,10 @@ func TestGenerateWithConfig(t *testing.T) {
 			Generated:   "generated.go",
 			ContextType: "github.com/Khan/genqlient/internal/testutil.MyContext",
 		}},
+		{"CustomContextWithAlias", "", nil, &Config{
+			Generated:   "generated.go",
+			ContextType: "github.com/Khan/genqlient/internal/testutil/junk---fun.name.MyContext",
+		}},
 		{"StructReferences", "", nil, &Config{
 			StructReferences: true,
 			Generated:        "generated-structrefs.go",