Remove build_plan from lockfile to improve perf

[f-f]

Fix #1262 by following the approach I explained at the time - we remove build_plan from each package of the lockfile since it's faster to recompute it than to decode it, as the files can get massive.

cc @finnhodgkin

[thomashoneyman]

This is a breaking change, in that workspaceLockCodec will no longer accept old lockfiles. I think the error Spago will emit is just "failed to decode lockfile, will regenerate a new one" which may be OK, but I'm thinking about --offline/--pure mode — is it the same error, but the lockfile doesn't get regenerated?

[thomashoneyman]

Code looks good, though 👍

[fsoikin]

There is also a comment that refers to storing all transitive dependencies in the lockfile:

spago/src/Spago/Command/Fetch.purs

Lines 162 to 164 in 782311b

	-- We compute the transitive deps for all the packages in the workspace, but keep them
	-- split by package - we need all of them so we can stash them in the lockfile, but we
	-- are going to only download the ones that we need to, if e.g. there's a package selected

I also noticed that there is a copy&paste of computing transitive deps for all workspace packages - here and here. Perhaps these should be consolidated?