osbuild2: new schema types: stages, inputs, sources

Adding new types and adapting copies of all the old types to match the
new Manifest schema:

New types:
- Stages
    - org.osbuild.ostree.init
    - org.osbuild.ostree.pull
    - org.osbuild.ostree.preptree (replaces org.osbuild.rpm-ostree)
    - org.osbuild.curl
- Converted from assemblers
    The concept of a Build and Assembler stage in gone now. Instead they
    are regular Stages like any other.
    - org.osbuild.oci-archive
    - org.osbuild.ostree.commit
- Sources
    - org.osbuild.curl
    - org.osbuild.ostree
- Inputs
    - org.osbuild.files
    - org.osbuild.ostree

Types with changes:
- Stages
    - org.osbuild.rpm:
        - New input structure for defining packages
        - New options

Basically copies:
- The rest simply rename the `Name` field to `Type`

Decoding types with interface fields:
Types that contain interfaces with multiple implementations implement
their own UnmarshalJSON method.  In these cases, we use a JSON decoder
with the `DisallowUnknownFields` option to catch errors during the
deserialization while trying to determine which implementation matches
the data.

Copied tests for copied types are adapted accordingly.

internal/osbuild2/source.go

@@ -0,0 +1,49 @@
+package osbuild2
+
+import (
+	"encoding/json"
+	"errors"
+)
+
+// A Sources map contains all the sources made available to an osbuild run
+type Sources map[string]Source
+
+// Source specifies the operations of a given source-type.
+type Source interface {
+	isSource()
+}
+
+type SourceOptions interface {
+	isSourceOptions()
+}
+
+type rawSources map[string]json.RawMessage
+
+// UnmarshalJSON unmarshals JSON into a Source object. Each type of source has
+// a custom unmarshaller for its options, selected based on the source name.
+func (sources *Sources) UnmarshalJSON(data []byte) error {
+	var rawSources rawSources
+	err := json.Unmarshal(data, &rawSources)
+	if err != nil {
+		return err
+	}
+	*sources = make(map[string]Source)
+	for name, rawSource := range rawSources {
+		var source Source
+		switch name {
+		case "org.osbuild.curl":
+			source = new(CurlSource)
+		case "org.osbuild.ostree":
+			source = new(OSTreeSource)
+		default:
+			return errors.New("unexpected source name: " + name)
+		}
+		err = json.Unmarshal(rawSource, source)
+		if err != nil {
+			return err
+		}
+		(*sources)[name] = source
+	}
+
+	return nil
+}