distro/rhel9: inline distribution structs

Inline the distribution structs in the common constructor.
Make the same changes that were made to RHEL 8:
- Remove major version argument: it's always 9
- Make the default constructor New() create the default minor version
  and rename it to plain RHEL 9.

The distroregistry now contains both rhel-9 and rhel-90, which point to
the same configuration but with different names.
The default rhel-9 should be updated to be an alias to rhel-91, the
current GA.

internal/distroregistry/distroregistry.go

@@ -19,7 +19,9 @@ var supportedDistros = []supportedDistro{
 	{fedora.NewF36, fedora.NewHostDistro},
 	{fedora.NewF37, fedora.NewHostDistro},
 	{fedora.NewF38, fedora.NewHostDistro},
+
 	{rhel7.New, rhel7.NewHostDistro},
+
 	{rhel8.New, rhel8.NewHostDistro},
 	{rhel8.NewRHEL84, rhel8.NewRHEL84HostDistro},
 	{rhel8.NewRHEL85, rhel8.NewRHEL85HostDistro},
@@ -27,7 +29,9 @@ var supportedDistros = []supportedDistro{
 	{rhel8.NewRHEL87, rhel8.NewRHEL87HostDistro},
 	{rhel8.NewRHEL88, rhel8.NewRHEL88HostDistro},
 	{rhel8.NewCentos, rhel8.NewCentosHostDistro},
+
 	{rhel9.New, rhel9.NewHostDistro},
+	{rhel9.NewRHEL90, rhel9.NewRHEL90HostDistro},
 	{rhel9.NewRHEL91, rhel9.NewRHEL91HostDistro},
 	{rhel9.NewRHEL92, rhel9.NewRHEL92HostDistro},
 	{rhel9.NewCentOS9, rhel9.NewCentOS9HostDistro},