builder: refactor job runner and use a shared semaphore across build jobs

Switching to a shared semaphore allows multi-build operations (compiler tests, package tests, etc.) to use the expected degree of parallelism efficiently.

While refactoring the job runner, the time complexity was also reduced from O(n^2) to O(n+m) (where n is the number of jobs, and m is the number of dependencies).

builder/library.go

@@ -43,7 +43,7 @@ func (l *Library) Load(config *compileopts.Config, tmpdir string) (dir string, e
 		return "", err
 	}
 	defer unlock()
-	err = runJobs(job, config.Options.Parallelism)
+	err = runJobs(job, config.Options.Semaphore)
 	return filepath.Dir(job.result), err
 }