switch to "src" layout #2381

Job	Run time
CI linters via Tox (pydocstyle)	40s
Schema Salad release test	7m 0s
CI tests via Tox (3, 12)	7m 17s
CI tests via Tox (3, 11)	8m 2s
CI linters via Tox (lintreadme)	33s
CI tests via Tox (3, 10)	8m 56s
CI tests via Tox (3, 13)	9m 3s
CI tests via Tox (3, 14)	4m 59s
Confirm that codegen typescript passes tests with CWL	47s
Confirm that codegen C++ passes tests with CWL	1m 40s
build_test_container	1m 41s
	50m 38s

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