Ensure we are not calling methods with side effects on the element graph in jobs

Description

This refactors our handling of elements to ensure we are never calling something with side effects while running in jobs.

This will become critical once we are running in a threaded scheduler, but is already important as it might hide some bugs (as some have already been found)

Benchmark

• Finished in: 91 mins, 23 secs
• Number of builders: [4, 8, 12]
• Target files: base-files/base-files.bst
• Number of runs: 3
• Number of warmups: 1
• Python versions: py38

action	python_version	commit	median time (secs)	mean time (secs) ± std
build - 12	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	174.92	177.78 ± 6.15
	py38	master - ea235b9d	172.71	173.10 ± 1.09
build - 4	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	178.4	177.31 ± 2.03
	py38	master - ea235b9d	172.44	172.83 ± 1.48
build - 8	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	173.01	172.82 ± 0.93
	py38	master - ea235b9d	170.19	170.81 ± 2.00
show	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	5.25	5.25 ± 0.15
	py38	master - ea235b9d	5.02	5.02 ± 0.06
show - cached	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	11.38	11.31 ± 0.27
	py38	master - ea235b9d	11.34	11.52 ± 0.34
show - sources fetched	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	7.42	7.32 ± 0.19
	py38	master - ea235b9d	7.28	7.26 ± 0.20
source fetch	py38	bschubert/stricter-job-scheduler-separation - 131d7b76	87	87.35 ± 0.63
	py38	master - ea235b9d	85.35	84.99 ± 0.86