WIP: Test junction replacements

This commit removes ostree to use it from GNOME SDK to use it from Freedesktop SDK instead. It also injects glib, gobject-introspection and libsoup into Freedesktop SDK graph.

This depends on https://gitlab.com/BuildStream/buildstream/-/merge_requests/1914 (and https://gitlab.com/BuildStream/buildstream/-/merge_requests/1913 in master)

It should also fix #200 (closed)

The main change is in elements/freedesktop-sdk.bst:

diff --git a/elements/freedesktop-sdk.bst b/elements/freedesktop-sdk.bst
index 3a060277..ad573ecd 100644
--- a/elements/freedesktop-sdk.bst
+++ b/elements/freedesktop-sdk.bst
@@ -6,3 +6,8 @@ sources:
 config:
   options:
     target_arch: '%{arch}'
+  replacements:
+    components/glib.bst: sdk/glib.bst
+    components/gobject-introspection.bst: sdk/gobject-introspection.bst
+    components/pygobject.bst: sdk/pygobject.bst
+    components/libsoup.bst: sdk/libsoup.bst