Unpack in existing directory if empty

Update issue template

Co-Authored-By: Mikhail Glushenkov <mikhail.glushenkov@gmail.com>

The pseudo-`NonEmpty` was temporarily introduced in
b4a96aab

in favour of the real `class Semigroup` provided by the canonical
`semigroups` package for GHC < 8.0

This way we can properly provide `Semigroup` instances as well a
`NonEmpty` lists in Cabal's public API in a uniform way for older GHCs
before GHC 8.0/base-4.9

NB: This incurs a dependency on `semigroups` *only* for GHC versions
    prior to GHC 8.0; similar to the pre-existing conditional
    dependency on the `fail` package

* Add default for v2-install
* Add changelog entry

Make a new bug report template.

I noticed that we need to ask whether it's `v1-` or `v2-` build. Maybe a template will help remove single roundtrip.

Fix #6082: Use pseudo-NonEmpty

Rename generate(s) to be greppable

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Simple.SrcDist: Fix search for DistPref

Previously we would look at directly at the sDistDistPref flag. However,
this may not be set, resulting in #6068. We should rather be using
findDistPrefOrDefault to find the DistPref directory.

Pass `@...` as a literal argument if the response file does not exist and support recursive response files

Update installing-packages.rst

Corrected a typo.

Improve error message for 'new-test' command

Fix spelling error

Suggest adding '--enable-tests'. [ci skip]

[ci skip]

Fix datadir path in generated `Paths_` file