fix infinite loop in D.Client.TargetSelector

[gelisam]

The pattern guard was clearly meant as a list-comprehension filter,
but on a let-bound pattern, it instead caused a circular loop in which
whether the LHS variables are bound the the RHS values depends on the
values of the LHS variables.

fixes #5081

---

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I tested this by running the problematic command from #5081. Result before the fix:

$ cabal new-build test:unknown-test-suite
cabal: Internal error in target matching. It should always be possible to find
a syntax that's sufficiently qualified to give an unambiguous match. However
when matching 'test:unknown-test-suite' we found test:unknown-test-suite
(unknown-component) which does not have an unambiguous syntax. The possible
syntax and the targets they match are as follows:
cabal: <<loop>>

Result after the fix:

$ cabal new-build test:unknown-test-suite
cabal: Internal error in target matching. It should always be possible
to find a syntax that's sufficiently qualified to give an unambiguous match.
However when matching 'test:unknown-test-suite' we found
test:unknown-test-suite (unknown-component) which does not have an unambiguous
syntax. The possible syntax and the targets they match are as follows:
'test:unknown-test-suite' which matches test:unknown-test-suite
(unknown-component), :pkg:test:lib:test:file:unknown-test-suite (unknown-file)

I also tested with all:test (typo for all:tests), which has similar results before and after the fix.

[alexbiehl]

This looks good to me! Let's merge this!

[23Skidoo]

Merged, thanks!

[23Skidoo]

Also cherry-picked into 2.2.