QMCPACK Update March 2020 - Part 2 (#15616)

* Add some comments explaining the choice of flag_handler.

* Fix QMCPACK install method.

* Add support for ppconvert. This requires a custom build method.

* Fix QMCPACK setup_run_environment. Nexus should be properly supported now.

* Cleaner way to check for intel-mkl in spec.

* Remove build method and use build_targets property instead.

* Additional fixed for install method. Effectively restoring the original install method.

* Add the missing backslash to fix directory names.

* Update var/spack/repos/builtin/packages/qmcpack/package.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Update var/spack/repos/builtin/packages/qmcpack/package.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Update var/spack/repos/builtin/packages/qmcpack/package.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Omit these conflicts on mkl variants for now, will hopefully be supportted with new concretizer in a couple of months.

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>