One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's CBOR field names, so tokens made before still decode. VariantKey
moves with the function that makes it. Tests follow the moved and
renamed code; the tests of the removed conversions go with them.

Model: opus-5-5

internal/server/fit_outside_internal_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 
 	"sneak.berlin/go/pixa/internal/encurl"
-	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // TestImageProxyRefusesFitOutsideAboveMaximum requests sizes with fit=outside
@@ -59,10 +59,13 @@ func TestImageProxyRefusesFitOutsideAboveMaximum(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		token, err := generator.Generate(&encurl.Payload{
-			SourceHost: upstreamHost, SourcePath: tt.path,
-			Width: tt.width, Height: tt.height, Format: imgcache.FormatJPEG,
-			FitMode: imgcache.FitOutside,
+		token, err := generator.Generate(&imagerequest.Request{
+			Source: imagerequest.Source{Host: upstreamHost, Path: tt.path},
+			Transform: imagerequest.Transform{
+				Size:    imagerequest.Size{Width: tt.width, Height: tt.height},
+				Format:  imagerequest.FormatJPEG,
+				FitMode: imagerequest.FitOutside,
+			},
 		})
 		if err != nil {
 			t.Fatalf("encurl Generate: %v", err)