We've added a path mapper to support simple mapping between path objects used
in the storage layer and the underlying file system. The target of this is to
ensure that paths are only calculated in a single place and their format is
separated from the data that makes up the path components.
This commit only includes spec implementation to support layer reads. Further
specs will come along with their implementations.
This commit adds regular expression definitions for several string identifiers
used througout the registry. The repository name regex supports up to five path
path components and restricts repeated periods, dashes and underscores. The tag
regex simply validates the length of the tag and that printable characters are
required.
Though we define a new package common, these definition should land in docker
core.
