bugfix: fix config merge order for OrderdDicts (#18482)

The logic in `config.py` merges lists correctly so that list elements
from higher-precedence config files come first, but the way we merge
`dict` elements reverses the precedence.

Since `mirrors.yaml` relies on `OrderedDict` for precedence, this bug
causes mirrors in lower-precedence config scopes to be checked before
higher-precedence scopes.

We should probably convert `mirrors.yaml` to use a list at some point,
but in the meantie here's a fix for `OrderedDict`.

- [x] ensuring that keys are ordered correctly in `OrderedDict` by
      re-inserting keys from the destination `dict` after adding the keys from
      the source `dict`.

- [x] also simplify the logic in `merge_yaml` by always reinserting
      common keys -- this preserves mark information without all the special
      cases, and makes it simpler to preserve insertion order.

Assuming a default spack configuration, if we run this:

```console
$ spack mirror add foo https://bar.com
```

Results before this change:

```console
$ spack config blame mirrors
---                                                          mirrors:
/Users/gamblin2/src/spack/etc/spack/defaults/mirrors.yaml:2    spack-public: https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
/Users/gamblin2/.spack/mirrors.yaml:2                          foo: https://bar.com
```

Results after:

```console
$ spack config blame mirrors
---                                                          mirrors:
/Users/gamblin2/.spack/mirrors.yaml:2                          foo: https://bar.com
/Users/gamblin2/src/spack/etc/spack/defaults/mirrors.yaml:2    spack-public: https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
```

0 files changed, 0 insertions, 0 deletions