Allow the following formats:
```yaml
mirrors:
name: <url>
```
```yaml
mirrors:
name:
url: s3://xyz
access_pair: [x, y]
```
```yaml
mirrors:
name:
fetch: http://xyz
push:
url: s3://xyz
access_pair: [x, y]
```
And reserve two new properties to indicate the mirror type (e.g.
mirror.spack.io is a source mirror, not a binary cache)
```yaml
mirrors:
spack-public:
source: true
binary: false
url: https://mirror.spack.io
```
Spack's source mirror was previously in a plain old S3 bucket. That will still
work, but we can do better. This switches to AWS's CloudFront CDN for hosting
the mirror.
CloudFront is 16x faster (or more) than the old bucket.
- [x] change mirror to https://mirror.spack.io
