* Create `pluginindex.Build` function
* Use `pluginindex` when no plugin URL provided for plugin add command
* Get more `plugin_add_command` BATS tests passing against Golang version
* Only use `pluginindex` when it is not disabled
* Update error message so it matches legacy format
* Add staticcheck.conf
* Move plugins/git package to /git.
* Update repoer interface in pluginindex package to match interface in git package
* Use git package's Repo struct in pluginindex package
* Switch to non-verbose test output in Makefile
* Create pluginindex package with PluginIndex struct with methods for updating index and retrieving plugin URLs
* Create IndexRepo interface to allow for easy testing out cloning real Git repository.
