Libraries packaged into hpi files are decided with the "nearest definition" rule of maven.
http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html

httpclient would be packaged into hpi files even though the plugin depends on apache-httpcomponent-client-4-api if the dependency declaration was inappropriate.

Developers have to know some rules to migrate to apache-httpcomponents-client-4-api-plugin:

• For plugins directly depending on org.apache.httpcomponents:httpclient:
  • Replace the dependency to org.apache.httpcomponents:httpclient with the dependency to org.jenkins-ci.plugins:apache-httpcomponents-client-4-api
• For plugins depending on libraries depending on org.apache.httpcomponents:httpclient:
  • Insert the dependency to org.apache.httpcomponents:httpclient BEFORE the dependency to the libraries.