Description of problem:
A third-party repository such as EPEL that wants to add a supplementary module stream to a module that is shipped by RHEL and enable a default profile for that stream will cause the defaults shipped by RHEL to be discarded.

Version-Release number of selected component (if applicable):
libmodulemd-2.5.0-2.el8

How reproducible:
Every time


Steps to Reproduce:
1. Pick a module shipped in RHEL 8 AppStream (e.g. "nodejs").
2. Create and build a new stream for that module (e.g "13").
3. Create a repository for the newly-built module with createrepo_c.
4. Produce a defaults document and append it to the repodata produced by the module build:

```
document: modulemd-defaults
data:
  module: nodejs
  modified: 201909240000
  profiles:
    11: [ "default" ]
```

(Make sure "modified" is a higher value than the one currently shipping in the RHEL AppStream repository.)


Actual results:
When the repository metadata from AppStream is merged with the additional repository, the defaults data from AppStream is lost, replaced by the one from the additional repository (or the reverse, if AppStream has a newer 'modified' value than the repo).


Expected results:
Non-conflicting profile defaults should be merged together. Conflicting profile defaults and default streams will be taken from the defaults document with the higher 'modified' value.

Additional info:
This bug exists in both the 1.x and 2.x APIs of libmodulemd.