Capture the repositories from source-repositories.

We try to expose metadata to users about what dib included in the
image and source-repositories was not doing that, so this fixes it. It
also permits easy introspection from other elements (such as an
upcoming PyPI cache layer).

Change-Id: I1e48e65f69ebe1f4556cc3bf35ccf195e99bcc76

elements/source-repositories/README.md

@@ -38,3 +38,7 @@ it will be renamed to \<destination\>
 If mulitple elements register a source location with the same <destination>
 then source-repositories will exit with an error. Care should therefore be taken
 to only use elements together that download source to different locations.
+
+The repository paths built into the image are stored in
+etc/dib-source-repositories, one repository per line. This permits later review
+of the repositories (by users or by other elements).

elements/source-repositories/extra-data.d/98-source-repositories

@@ -88,7 +88,9 @@ function get_repos_for_element(){
                 return 1
                 ;;
             esac
-
+            # Capture the in-instance repository path for later review / other
+            # elements (like a pypi dependency cache).
+            echo "$REPOPATH" | sudo dd of=$TMP_MOUNT_PATH/etc/dib-source-repositories oflag=append conv=notrunc
         else
             echo "Couldn't parse '$line' as a source repository"
             return 1