webpack: define and wrangle the difference between lockdown and lavamoat options.

[davidmurdoch]

The webpack build doesn't yet support lavamoat, but when it does we'll need to figure out why (in the gulp build) only some lockdown files are loaded when lavamoat is used.

[davidmurdoch]

from #26256:

The lavamoat plugin will be responsible for injecting ses and calling lockdown. We need to figure out if we want to remove the --lockdown option from webpack. I vote for complete removal of the option (effectively making --lavamoat the same as doing --lavamoat --lockdown).

We might need to tweak how lockdown-more.js is injected into the bundle, as well as how snow and use-snow are bundled. Wrapping these in lavamoat runtime would cause problems.