Change interpretation of the jobs argument?

[wlandau]

From #435 (comment), I am starting to think make(jobs = 16) should be equivalent to make(jobs = c(imports = 1, targets = 16)) rather than the current make(jobs = c(imports = 16, targets = 16)). What do you think? cc @kendonB, @idavydov.

[wlandau]

Should be a simple matter of changing this line in parse_jobs() and updating this documentation.

[idavydov]

In my opinion that would be a good pragmatical decision.

1. In case of small plans, this should not affect the performance significantly.
2. For the large plans with a large number of jobs this should provide significant benefit (see Huge overhead in adjacent_vertices in a moderately sized project #435 (comment)).
3. The only case when this could make things worse is a huge plan with the moderate number of workers (e.g. 1k targets, 5 jobs).

But I think cases when imports are the bottleneck should not be common.

[wlandau]

Yeah, I agree. And the decision is much easier now that version >= 6.0.0 has far fewer imports (ref: #519).

[wlandau]

Edit: please see b786fe3.100c71f has a bug.