Officially support `checksums` EC parameter being a dict

[Flamefire]

https://docs.easybuild.io/en/latest/Writing_easyconfig_files.html#common-easyconfig-param-sources-checksums documents the checksums parameter as a list of checksums

However the implementation (partially) supports it being a dict

This seems to be currently not really supported as the CHECKSUMS type verification doesn't include that case and to_checksum does neither.

So we should either officially support that or remove the code in get_checksum_for

[Flamefire]

Requires resolution of

• enhance documentation of checksums easyconfig parameter easybuild-docs#104