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 short CBOR field names. 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/imgcache/pendingcounts_internal_test.go

@@ -6,6 +6,8 @@ import (
 	"errors"
 	"testing"
 	"time"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // pendingCountsWait is how long a test waits for the database's connection,
@@ -92,7 +94,7 @@ func TestService_Get_ReturnsByItsDeadlineWhileTheDatabaseIsBusy(t *testing.T) {
 
 	deadline, _ := ctx.Deadline()
 
-	results := startGet(ctx, svc, photoVariant(fixtures, 85, FitCover))
+	results := startGet(ctx, svc, photoVariant(fixtures, 85, imagerequest.FitCover))
 
 	// The request has made its database reads by the time it fetches.
 	select {