Fix handling of default sources

[riccardoporreca]

Resolves #231, also

• making sure to handle separately the case of unspecified --file arguments from the case when --file environment.y(a)ml is explicitly used
• reviewing the checks for unspecified sources, also covering the usage of the defaults as fall-back when the lockfile exists

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[riccardoporreca]

The failure on Windows seems to be for a "flaky" test, but I cannot re-run the job on this repo

[riccardoporreca]

On hold waiting for PR #204, see #204 (comment)

[riccardoporreca]

@maresb, similar to #230, I would be happy to rebase this PR as #231 is still an issue with the current code-base.

Let me know if I can go ahead with this or I should rather wait for some ongoing work-in-progress to be first merged to the main branch.

[maresb]

Sorry, no time to look at this in any detail this week, but generally speaking, if the issue isn't resolved, I'd recommend just going for the fix. We'll have to deal with any merge conflicts as they arise, likely via the greedy algorithm. Thanks a lot for all the work!!!