Make Jacoco and Cobertura plugins no longer required

[kageiit]

Have the ability to parse and report coverage from cobertura/jacoco xml reports natively in the phabricator jenkins plugin. Currently only cobertura is supported (in the release version of the plugin). Even when the cobertura plugin is applied, we still process all the coverage xml files to collect line coverage. This change will completely remove any reliance on external coverage jenkins plugins and process coverage xml for cobertura and jacoco natively.

Since the jacoco coverage currently available on master has not be released yet, it will be refactored to remove the bits that are reliant on the jacoco plugin and the parsing of the jacoco exec binary format

[kageiit]

cc @ChaitanyaPramod @artms