Factor out logic about how pgroups are assigned

Introduce pgroup_provenance_t, a type which captures "where the pgroup
comes from." This centralizes some logic around how pgroups are
assigned, and it anticipates concurrent execution.

src/parse_execution.cpp

@@ -1302,6 +1302,9 @@ end_execution_reason_t parse_execution_context_t::run_1_job(tnode_t<g::job> job_
 
     // Clean up the job on failure or cancellation.
     if (pop_result == end_execution_reason_t::ok) {
+        // Set the pgroup assignment mode, now that the job is populated.
+        job->pgroup_mode = get_pgroup_provenance(job, lineage);
+
         // Success. Give the job to the parser - it will clean it up.
         parser->job_add(job);