Fold *systems-being-defined* into the *asdf-cache*.
Add regression test for no infinite loop when processing asd files that mutually define each other's systems. (Prompted by Robert Strandh inquiring about the bug fixed in 2.015. after my mentioning an infinite loop in my ASDF3 article.) Checked that removing the set-asdf-cache-entry in parse-defsystem triggers the bug.
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