gvplugin_load: use a 'strview_t' for 'reqpkg'

gvplugin_load: use a 'strview_t' for 'reqpkg'

This string is always terminated by `'\0'`, not `':'`, so there is little
advantage in using a `strview_t` over a `const char*`. But upcoming changes will
refactor the surrounding code to use `strview_t`, so it will be more readable to
use `strview_t` consistently here.