maktaba should detect correct plugin name for github-USER-PLUGINNAME/ on rtp

[dbarnett]

Similar to #112, maktaba#plugin#Get and related functions like maktaba#library#Require will detect the wrong name for plugin dirs named like github-USER-PLUGINNAME/, which is a convention that VAM uses. For instance,

VAMActivate github:ConradIrwin/vim-bracketed-paste

adds a plugin dir github-ConradIrwin-vim-bracketed-paste/ to vim's runtimepath, and maktaba uses the whole dir name "github-ConradIrwin-vim-bracketed-paste" as the name of the plugin, whereas bracketed-paste is the actual plugin name that should be used.

Most of these plugins don't have an addon-info.json, so adding more heuristics to maktaba#plugin#CanonicalName would be the only way to determine the right plugin name (otherwise the addon-info.json name would take precedence).