package/eigen: use cmake-package infra

package/eigen: use cmake-package infra

The eigen package was introduced as a generic package, but upstream was
in fact using CMake.

The motivation of this change is to fix package detection with CMake.

Eigen3 library normally installs a signature file named
"signature_of_eigen3_matrix_library" to help library detection:
https://gitlab.com/libeigen/eigen/-/blob/3.3.7/CMakeLists.txt#L423

The library also provide a CMake macro that use this file to
detect it:
https://gitlab.com/libeigen/eigen/-/blob/3.3.7/cmake/FindEigen3.cmake#L76

Without the signature file installed, packages searching for this
library with this method will fail. Other packages usings pkg-config are
not affected by this issue.

By using the cmake-package infrastructure, all the needed files
are now installed, fixing this CMake detection issue.

Other changes in this patch:
- Updated the Eigen git repository to the new url:
  https://gitlab.com/libeigen/eigen
- Removed all build and install staging commands
  (now included in upstream cmake)
- Package needs EIGEN_SUPPORTS_IN_SOURCE_BUILD = NO
- Removed the BR2_PACKAGE_EIGEN_UNSUPPORTED_MODULES option,
  as this option is not proposed by the upstream CMake.
  Unsupported module header files are now unconditionally installed. As
  such, no need to introduce a legacy entry for
  BR2_PACKAGE_EIGEN_UNSUPPORTED_MODULES: users that had it enabled will
  still get the files installed, while those that did not will get them
  installed now.
- Updated hash for source package, because the first component in the
  stored paths changed from eigen-eigen-323c052e1731/ to eigen-3.3.7/
  and some mercurial related files (.hg_archival.txt, .hgtags) got
  dropped after the conversion to git.
- Reformat hash file with two spaces delimiters
- Define EIGEN_CONF_OPTS to set pkg-config .pc install path

Signed-off-by: Julien Olivain <juju@cotds.org>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>