if there is more than one qt5 attr and this file is copied then the arg
to generateSplicesForMkScope should be changed

https://github.com/NixOS/nixpkgs/blob/master/doc/builders/special/makesetuphook.section.md

Without this, two different copies of qtbase to wind up in the
buildInputs for cross compiled builds, leading to this sort of
breakage for PyQt:

```
Error: detected mismatched Qt dependencies:
     /nix/store/8g7xc01pj6y569qfyf5ccfbs5dr87wwq-qtbase-aarch64-unknown-linux-gnu-5.15.9-dev
     /nix/store/w19n5gzlx4fv63a3dn5965s661nhakv9-qtbase-5.15.9-dev
```

This commit adds the `-device` and `-device-option` flags necessary
for cross compilation to `configureFlags`.

It also adds the (one-entry at the moment) Rosetta Stone for QT-5 as
a `let`-defined `qtPlatform` function which takes a nixpkgs platform
and returns a QT-5 `mkspecs`-string.

This commit:

- sets the `PKG_CONFIG_SYSROOT_DIR` and `PKG_CONFIG_LIBDIR`
  environment variables when compiling a cross-targeted `qmake`.
  This is required; if these environment variables are unset,
  `qmake` won't even try to use `pkg-config`.

- writes `LDFLAGS`, `CFLAGS`, and `CXXFLAGS` into `qmake.conf`.

QT's configure script doesn't understand these, so let's not pass them.

Later stages of the build process expect to find

  $bin/bin/{qmlformat,qmltyperegistrar}

however qt's quirky build tooling doesn't feel like it's in the mood
to put those files there when doing a cross-compiled build.  Instead
it puts them in $dev/bin.  Let's move them to the right place and
add symlinks from whence we moved them (the symlinks can't point the
other way due to reference cycles).

QtWebChannel does not produce binaries when it is cross-compiled.
Let's omit the `$bin` output in that case.

The expression in its current form attempts to invoke the python2
interpreter for the wrong (i.e. build) architecture.

Using python3 explicitly, rather than relying on aliases, fixes this.

When cross-compiling, qmake insists on being able to find a
`pkgsHostForTarget`-style `pkg-config` with the name `pkg-config`
(no target prefix allowed).

To keep `qmake` happy, this commit creates a one-symlink derivation
containing a link `pkg-config` which points to
`${targetPrefix}pkg-config`.

This commit also sets the `PKG_CONFIG` environment variable.

The build logs from PyQT5 are quite verbose; it's nice to trim them
down so all you see is the errors.  This commit adds an option which
does that.

This commit causes pyqtwebengine to build using -j$NIX_BUILD_CORES,
and also adds the verbose flag which really means "don't send build
errors to /dev/null".

This commit:

- adds qtwebchannel to deps
- takes sip from buildPackages

This commit successfully cross compiles qutebrowser and all its
dependencies (pyqt, qt-5, wayland, etc).