Calculate package digest on first use.

[fschulze]

This allows for faster startup times in case no template is used.

Helps with things like argcomplete or faster responses for errors that happen before templates are used. In my project the calculation of pkg_digest takes almost one second. It might be possible to use something different then packages_distributions from importlib.metadata, which is the slow part, but I couldn't find a suitable replacement.

[coveralls]

Pull Request Test Coverage Report for Build 12558875756

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 92.36%

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Totals
Change from base Build 12455853721:	0.03%
Covered Lines:	3920
Relevant Lines:	4170

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💛 - Coveralls