cli/command/manifest: remove redundant uses of ParseRepositoryInfo

[thaJeztah]

cli/command/manifest: remove redundant uses of ParseRepositoryInfo

ParseRepositoryInfo parses an image reference and returns information
about the Repository and the registry. As part of this, it validates if
the registry's hostname is considered valid using ValidateIndexName,
as well as normalizing the image reference to strip tags and digests
using reference.TrimNamed.

ValidateIndexName only provides very limited value; the only validation
happening is to check for the hostname to not start, or end with a hyphen.

The cli/command/manifest package used ParseRepositoryInfo in various
locations where only the repository name was used (i.e., the result
of reference.TrimNamed on the given reference), and in one location
only used it to validate the registry name.

For buildPushRequest, the call was fully redundant, as RepoNameForReference
was used on the result, calling newDefaultRepositoryEndpoint, which
uses ParseRepositoryInfo internally, so we were only repeating that work.

This patch removes uses of ParseRepositoryInfo in those places, and instead
calling reference.TrimNamed directly.

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[codecov-commenter]

Codecov Report

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

Project coverage is 59.33%. Comparing base (fe0a8d2) to head (55a83af).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5877      +/-   ##
==========================================
+ Coverage   59.32%   59.33%   +0.01%     
==========================================
  Files         353      353              
  Lines       29748    29731      -17     
==========================================
- Hits        17647    17640       -7     
+ Misses      11113    11107       -6     
+ Partials      988      984       -4