cache-url: Give up on curl install for Redhat platforms

The RedHat platforms vary if they come pre-installed with curl or
curl-minimal.  For example, Fedora 37 container images have curl, and
centos 9-stream and Rocky images have curl-minimal.  If you try and
install curl when curl-minimal is installed, you get an error, and
vice-versa.

Unfortunately package-installs can't really sort this out; we're just
passing a package list to the system package manager.  We don't have a
way to say "the curl OR curl-minimal package is fine".  As this breaks
builds and is such a common dependency that it's already there, let's
just add a note that curl is required and blank out the package-map.

Change-Id: I9ccebe2dbf3a8682dab60c2070c5f78849e01446

diskimage_builder/elements/cache-url/README.rst

@@ -1,4 +1,10 @@
 =========
 cache-url
 =========
+
 A helper script to download images into a local cache.
+
+**NOTE** : on RedHat platforms, ensure the curl binary is available.
+Due to conflicting differences in platform usage of ```curl-minimal``
+and ```curl``, the usual package dependency methods do not work for
+this package.

diskimage_builder/elements/cache-url/pkg-map

@@ -1,4 +1,9 @@
 {
+    "family": {
+        "redhat": {
+            "curl": ""
+        }
+    },
     "distro": {
         "gentoo": {
             "curl": "net-misc/curl"