use local ConvertToHostname() implementation

[thaJeztah]

Commit 27b2797 (#1642) added a local implementation of this function, so let's use the local variant to (slightly) reduce the dependency on moby's registry package.

Also made some minor cleanups.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.78%. Comparing base (925e7d6) to head (8376b3e).

Additional details and impacted files

@@ Coverage Diff @@## master #3599 +/- ##
==========================================
+ Coverage 61.76% 61.78% +0.01% 
==========================================
Files 297 297 Lines 20768 20768 ==========================================
+ Hits 12828 12831 +3 + Misses 7024 7023 -1 + Partials 916 914 -2 

[thaJeztah]

@rumpl@ndeloof ptal 🤗

[ndeloof]

I don't think "Copied from github.com/docker/docker/registry.ConvertToHostname to reduce dependencies" makes our codebase better.

1. we still (obviously) depend on docker/docker as the API is not an isolated project vs engine backend
2. we now have two implementations of the same ConvertToHostname function to maintain (for sure, low maintenance expected here)

imho it would make more sense for such utility func to be shared between cli and engine within one of the api package. I'm not sure about the initial intent in #1642