c/image/docker depends on compression implementations

[mtrmac]

… via pkg/compression, primarily because it refers to algorithm names, see e.g. openshift/oc#737 .

This should be possible to resolve by making the algorithm names string constants in pkg/compression/types, and using these constants both in pkg/compression and in manifest (e.g. compressiontypes.GzipAlgorithmName instead of compression.Gzip.Name()), as well as using pkg/compression/types.Algorithm instead of pkg/compression.Algorithm.

[mtrmac]

This has become much worse since:

• pkg/compression drags in all of c/storage, including graph drivers (EDIT: Fixed in Reduce the dependency of pkg/compression on c/storage/pkg/chunked #1312 )
• the blob reuse logic, via the need to set types.BlobInfo.CompressionAlgorithm, causes c/image/docker to depend on pkg/compression.

[mtrmac]

• the blob reuse logic, via the need to set types.BlobInfo.CompressionAlgorithm, causes c/image/docker to depend on pkg/compression.

This now seems impossible to fix. We could modify the internal-only TryReusingBlob… API to return only a name, making the transports independent, but then the public TryReusingBlob, which is documented to set CompressionAlgorithm, would have to, I guess, just give up and not do any substitutions… which would be an API break as well.