wic: Avoid problems with "-" characters in plugin names

wic: Avoid problems with "-" characters in plugin names

Remap "-" characters in plugin names to "_" so wic plugins
can be extended using standard python class inheritance.

This change means wic files can be incrementally updated over time
to the correct name rather than breaking everything. Actual plugin
module files will need to be renamed as done in previous patches.

Also remove a double call to get_plugins() which isn't needed.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>