sources: rename org.osbuild.{files -> curl}

The `org.osbuild.files` source provides files, but might in the
future not be the only one that does. Therefore rename it to
match the internal tool that is being used to fetch the files.
This is done for most other osbuild modules that target tools.

The format v1 loader is adapted to make this change transparent
for users of the v1 format, so we are backwards compatible.

Change the MPP depsolve preprocessor so that for format v2 based
manifest `org.osbuild.curl` source is used. Also rename the
corresponding source test. Adapt the format v2 mod test to use
the curl source.

test/mod/test_fmt_v2.py

@@ -12,7 +12,7 @@ import osbuild.meta
 BASIC_PIPELINE = {
     "version": "2",
     "sources": {
-        "org.osbuild.files": {
+        "org.osbuild.curl": {
             "items": {
                 "sha256:6eeebf21f245bf0d6f58962dc49b6dfb51f55acb6a595c6b9cbe9628806b80a4":
                 "https://internet/curl-7.69.1-1.fc32.x86_64.rpm",