This extends Spack functionality so that it can fetch sources and binaries from-, push sources and binaries to-, and index the contents of- mirrors hosted on an S3 bucket.

High level to-do list:

- [x] Extend mirrors configuration to add support for `file://`, and `s3://` URLs.
- [x] Ensure all fetching, pushing, and indexing operations work for `file://` URLs.
- [x] Implement S3 source fetching
- [x] Implement S3 binary mirror indexing
- [x] Implement S3 binary package fetching
- [x] Implement S3 source pushing
- [x] Implement S3 binary package pushing

Important details:

* refactor URL handling to handle S3 URLs and mirror URLs more gracefully.
  - updated parse() to accept already-parsed URL objects.  an equivalent object
    is returned with any extra s3-related attributes intact.  Objects created with
    urllib can also be passed, and the additional s3 handling logic will still be applied.

* update mirror schema/parsing (mirror can have separate fetch/push URLs)
* implement s3_fetch_strategy/several utility changes
* provide more feature-complete S3 fetching
* update buildcache create command to support S3

* Move the core logic for reading data from S3 out of the s3 fetch strategy and into
  the s3 URL handler.  The s3 fetch strategy now calls into `read_from_url()` Since
  read_from_url can now handle S3 URLs, the S3 fetch strategy is redundant.  It's
  not clear whether the ideal design is to have S3 fetching functionality in a fetch
  strategy, directly implemented in read_from_url, or both.

* expanded what can be passed to `spack buildcache` via the -d flag: In addition
  to a directory on the local filesystem, the name of a configured mirror can be
  passed, or a push URL can be passed directly.