Some of the Go tests actually cloned remote repos, which meant slow tests, and tests failures if I was working offline. These changes allow all Go tests to run offline. The test code now clones local Git repos instead of remotes ones.
* Add otiai10/copy as a dependency
* Create repotest package for test code that needs to work with Git repos
* Add back missing test script for execute package
* Update git, pluginindex, and plugins package tests so they work offline
* 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
