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/
```
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