Fix `check_checksums` when using templates, patch-dicts or checksums.json

[Flamefire]

This is a similar fix as https://github.com/easybuilders/easybuild-easyconfigs/pull/15973

check_checksums had various shortcomings, some touched by https://github.com/easybuilders/easybuild-framework/commit/7112e406dc8ee75fd3d5257648d23d801d77e813 (from @boegel ) which disabled templating entirely when getting the exts_list list.

This breaks when checksums.json is used: It will report missing checksums even when they are available in the checksums.json
Additionally templates in the name/version of extension won't be resolved causing the error message harder to understand

A quick fix was to replace the above commit by allow_unresolved_templates-decorator to allow partially resolved extensions but don't fail as before when templates are used e.g. in install_cmd which fixes the issues with the name/version of the extension

However that resolves too deep as inside the extension options different template values must be used.

We have collect_exts_file_info which does handle all those peculiarities of the extension specs already so just use that.

This also handles the default file name of extension sources and nosource which we can rely on here if it fills sources correctly. This requires #4054 (merged here so that tests pass)

It also fixes a failure when patches were specified as dicts as In sources the keyname is "filename" while in patches it is "name" which causes a KeyError in check_checksums_for

[Flamefire]

@boegel This might be useful to include in the release as it fixes multiple subtle issues in how checksums are handled in the checks. Especially by replacing duplicated logic by collect_exts_file_info making it a lot more reliable.

A big chunk is just refactoring to avoid duplication, see individual commits.

This code is heavily exercised on easyconfigs CI so running a few PR CIs after merging this should discover any hidden issues that the added tests didn't catch.