kwin-fourier, qt6-base-fourier: detach upstream-tarball name from pkgname

After the rename, source URLs and extracted-dir refs that used
$pkgname-$pkgver were producing kwin-fourier-6.6.4.tar.xz etc. which
KDE doesn't ship. Use a separate _upname/hardcoded value for the
upstream artifact.

  kwin-fourier:  introduce _upname=kwin, use everywhere the upstream
                 dir/tarball name is referenced (source URL, prepare
                 patch -d, cmake -S)
  qt6-base-fourier: $_pkgfn=${pkgbase/6-/} produced 'qtbase-fourier'
                 with the rename. Hardcode _pkgfn=qtbase.

arch/qt6-base-fourier/PKGBUILD

@@ -108,7 +108,7 @@ optdepends=('freetds: MS SQL driver'
             'postgresql-libs: PostgreSQL driver'
             'unixodbc: ODBC driver')
 groups=(qt6)
-_pkgfn=${pkgbase/6-/}
+_pkgfn=qtbase
 source=(git+https://code.qt.io/qt/$_pkgfn#tag=v$_pkgver
         qt6-base-cflags.patch
         qt6-base-nostrip.patch