add support for checksums specified in external checksums.json file

[mboisson]

Renamed the branch since we switch from yaml to json.
This replaces #3748
to address
#3746

• no test implemented
• not sure what to do with --inject-checksums (added --inject-checksums-to-json)

I have tested this PR with Python-3.8.2-GCCcore-9.3.0.eb by removing the checksums from the EasyConfig, and with --enforce-checksums enabled.

It allows to remove checksums both for the main file and for the exts_list.

[mboisson]

So far, I have assumed that one had either checksums in the EasyConfig file or a checksums.json file. The checksums.json file mechanism only takes effect if there are no checksums in the EasyConfig file.

This is not really useful to reuse existing EasyConfigs with a bump --try-software-version, as the list provided in the EasyConfig will take precedence and the build will fail.

I am rewriting the code such that both the checksums from the EasyConfig and the checksum from the Json file are loaded, and then providing a build_option to guide which one should be prioritized to validate the checksum if there are both.

[mboisson]

@boegel, do you think we need to address --new-pr and the --inject-checksums-to-json when using --try-software-version issue in this PR ?

[boegel]

@mboisson Some proposed cleanup for the test part of this in ComputeCanada#26

I intend to make a pass over the non-test part too to propose some minor improvements, but this looks really good overall, thanks for all the effort you've put into this already! 👍

[boegel]

lgtm