Since bundled plugins will be removed in an upcoming version of Packer,
this commit adds a new warning message whenever a template uses one such
plugin.
This warning has been implemented on build, validate, console and the
inspect subcommands.
In addition to warning about the upcoming change and potential issue
this will cause, this warning message proposes solutions to the user so
they know what they'll have to do in order not to rely on those bundled
plugins later.
Lucas Bajolet
a2930bda4f