license-report: Support prefix

[hseg]

In trying to figure out how to use license-report for packaging, I've run into
the problem that typically, one wants to write the report to a different path
than where it will end up on the end-user's machine.

In particular, the links to the dependencies' licenses will break if the report
is moved out of licensedir.

Three solutions suggest themselves:

• Add another path argument prefix that will be prepended to licensedir when
  it comes to writing the dependencies' licenses (i.e. the report will link to
  the licenses as if they're saved under licensedir, but they actually will be
  saved to prefix/licensedir)

• Use relative paths in the report. To avoid desynchronization, perhaps have
  licensedir imply writing the report to licensedir/$target.md or
  licensedir/index.md?

• Do nothing, and have the end-user fudge the paths. One possible way to do this
  (note the readarray targets line is due to
  Add a cabal target command haskell/cabal#9744)

  licensedir="$prefix/usr/share/licenses/$pkgname/"
  install -Dm644 LICENSE* -t "$licensedir"
  mkdir "$licensedir/dependencies"
  readarray -t targets < <(jq -r '."install-plan"[] |
      {style, target: (
          ."component-name" +
              if ."component-name" | contains(":")? | not then
                  ":\(."pkg-name")" else "" end)
      } | select(.style == "local") | .target')
  for trg in "${targets[@]}"; do
      tmpprefix="$(mktemp -dp .)"
  
      pushd "$tmpprefix"
      cabal-plan license-report --licensedir . > "$trg.md"
      popd
      # alternatively, if absolute paths are desired
      cabal-plan license-report --licensedir "$tmpprefix" | \
        sed "s#$tmpprefix#/usr/share/licenses/$pkgname/#" \
        > "$tmpprefix/$trg.md"
  done
  cp -r "$tmpprefix"/* "$licensedir"