This will fix if a package will be upgraded from some extras to all
extras. Previously, it will not work because some dependencies of 'all'
are already installed, so the feature 'all' is already installed.
Now, it will use a list of all extras to iterate over them separatly. This
will result it will never install all extras via `[all]`, but rather
`[foo,bar]`.
As the previous detection took the wrong values, this explorer now
checks if packages for an extra are installed or not. If not, the extra
is not installed.
Based on the information of the explorer, it will install the package
again with the absent extras.
matze