bug#41451: Haskell packages retain references to bootstrap variants

[Ricardo Wurmus]

Some Haskell packages have a “-bootstrap” variant to cut dependency
cycles.  Unfortunately, these bootstrap variants remain in the reference
graph alongside their non-bootstrap counterparts.

An example:

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$ guix gc -R /gnu/store/1k0dqlyqyvhd3ax60a2y6l2sip0z7b26-ghc-scientific-0.3.6.2 
| grep -- -bootstrap
/gnu/store/lvas21ha80yp02lc3z8fgpcaf59n1jp7-ghc-unordered-containers-bootstrap-0.2.10.0-doc
/gnu/store/22309kdxaapz6z3jm6wfy6bk1yybvxg9-ghc-hashable-bootstrap-1.2.7.0-doc
/gnu/store/mhpfjyx302d89w9484ivylczp8pzzfl2-ghc-hashable-bootstrap-1.2.7.0
/gnu/store/bfc5iyrlqr23fhp8awmd481za4aank9h-ghc-nats-bootstrap-1.1.2
/gnu/store/1nym2bjr3adhy1pa79iiasnfdz285b08-ghc-unordered-containers-bootstrap-0.2.10.0
/gnu/store/xjgjshwhpc36nfagfz0bg3nrg6fkn615-ghc-integer-logarithms-bootstrap-1.0.3
/gnu/store/jljsapda2vbxn7p37i6sk026r9gbrib7-ghc-scientific-bootstrap-0.3.6.2
/gnu/store/gjsmr0nrgsdyxcq9wizcx31cxpg7f9x7-ghc-semigroups-bootstrap-0.18.5
/gnu/store/ywlnsg1lcf0yzisivi7gm2pmls5jnxbb-ghc-attoparsec-bootstrap-0.13.2.3
/gnu/store/w0q1s85mwvxk44gcs9lmh994rm8p7ik6-ghc-wcwidth-bootstrap-0.0.2
/gnu/store/ibk4x2bkqm7krc6cc2d61hvi213zvvfq-ghc-splitmix-bootstrap-0.0.3
/gnu/store/57zjc7i8hb9sb18yflr46yi1vvr21y31-ghc-clock-bootstrap-0.8
--8<---------------cut here---------------end--------------->8---

-- 
Ricardo