Fix to_checksums with None values in dicts and recursion

[Flamefire]

Having a 'src': None entry in a dict for checksums is as valid as having a None entry directly in the list. However the current function didn't handle it and crashed.
Fix that as well as a few corner cases especially in the recursive case by introducing a new function for handling checksum entries in the checksum list and limiting the recursiveness.

It will now throw an EasyBuildError similar to e.g. to_dependency instead of trying to continue and fail at a random other place.
I extended the testcases to cover the original issue (None as a value in a dict) and some more correct and error cases

Please carefully review and check if you can come up with any valid or invalid checksums entry that might break using the new function. Especially regarding the YEB compatibility this was pretty hard as for lists of strings you cannot know if those are 2 checksums without a type that should both match or a checksum and type. In EC-files (Python) one can use a tuple for that but then again it is hard to tell if that tuple is a checksum and type or 2 alternative checksums.

Fixes #4142

• Requires Fix the checksum type check #4164 (contained here)
• enhance documentation of checksums easyconfig parameter easybuild-docs#104

[Flamefire]

I traced the failure to a difference in YEB and EB formats and the validator which seemingly has a bug:

The toy-0.0.* contains a single source with alternative checksums:

checksums: [[
    'be662daa971a640e40be5c804d9d7d10',  # default [MD5]
    '44332000aa33b99ad1e00cbd1a7da769220d74647060a10e807b916d73ea27bc',  # default (SHA256)
    ['adler32', '0x998410035'],
    ['crc32', '0x1553842328'],
    ['md5', 'be662daa971a640e40be5c804d9d7d10'],
    ['sha1', 'f618096c52244539d0e89867405f573fdb0b55b0'],
    ['size', 273],
]]

The difference is:

• YEB: `[['be...', '44...', [...], [...], [...], [...], [...]]]``
• EC: [['be...', '44...', (...), (...), (...), (...), (...)]]

I.e. the EC correctly uses tuples while the YEB uses lists. However actually both are wrong and need to be converted because alternative checksums must be specified as a tuple:

easybuild-framework/easybuild/tools/filetools.py

Line 1291 in 8ba299f

elif isinstance(checksum, tuple):

However the current definition of the checksums type is:

CHECKSUM_LIST = (list, as_hashable({'elem_types': [str, tuple, STRING_DICT]}))
CHECKSUMS = (list, as_hashable({'elem_types': [str, tuple, STRING_DICT, CHECKSUM_LIST]}))

I.e. the we wrongly allow a list where we expect a tuple.
Furthermore this doesn't cover all possible use cases (e.g. dicts with None entries or alternatives) and allows wrong entries, e.g. tuples of any types. As I don't see that it is possible to express all valid entries I'd purposely restrict this to less than is possible and make use of to_checksums to do the final verification.

[boegel]

@Flamefire This needs another sync with develop after merge of #4255