use SYSTEM constant for dependency that uses system toolchain in dumped easyconfig

[boegel]

Currently, we usually using True as 4th value in a dependency specification if the system toolchain should be used, for example:

dependencies = [('CUDA', '11.4.1', '', True)]

A more intuitive alternative, which has actually been silently supported already for a long time, is to use the SYSTEM constant instead (since EasyBuild v4.0.0, see #2877):

dependencies = [('CUDA', '11.4.1', '', SYSTEM]

This translates to the following under the covers:

dependencies = [('CUDA', '11.4.1', '', {'name': 'system', 'version': 'system'}]

Directly specifying the toolchain to use for a dependency through the classic name/version dict value has been supported for ages (at least since EasyBuild v2.7.0, but also before already); the SYSTEM constant just makes this (significantly) less ugly.

This practice currently isn't adopted in the central easyconfigs repository though, mainly because the easyconfigs test suite in which we check whether the values of easyconfig parameters for the original easyconfig and for the easyconfig file that's obtained by dumping the parsed easyconfig are equal.
This check fails when using the SYSTEM constant in a dependency specification as shown above, because the dumped easyconfig will use the more cryptic True value instead.

With the changes in this PR, we will be able to use SYSTEM rather than True to indicate that the system toolchain should be used for a dependency for easyconfigs being contributed to the central easyconfigs repository...

Note that this will need a change in the easyconfigs test suite analogous to the workaround that @ocaisa added to easybuilders/easybuild-easyconfigs#15900 though, since the check on the toolchain part of a dependency will fail if True is still being used instead of the SYSTEM constant (which we should keep supporting for now, we can consider deprecating that later, and start requiring that new PRs use SYSTEM rather than True at some point).

[boegel]

Necessary change for easyconfigs test suite is implemented in easybuilders/easybuild-easyconfigs#16126, which should be merged ASAP after this PR gets merged...

[Micket]

lgtm