simplify loop in collect_exts_file_info

[Flamefire]

The loop over exts_list uses an excessive amount of branches and hence indentation making it hard to read.

We have 3 simple cases:

1. Extension is a string -> use it as the name and do nothing else
2. extension is neither a string, list nor dict -> Error out
3. extension is a list of only a name -> use as the name and do nothing else

Moving the type check to the top of the loop makes it instantly visible which types are accepted, the elifs at the bottom are hard to find after all the indents.

Using continue after handling the simple cases reduces the indentation by 2 levels and makes it clear, that nothing else is done for those.

[boegel]

@Flamefire I changed to target branch in this PR from 5.0.x to develop, you should synchronize your PR branch with current develop branch (which has received a massive update after the release of EasyBuild v5.0.0, see #4825)