refactor: remove uses of Option in new_with_config()

[pickx]

so this PR is rather large in terms of LoC, and I hope this is something you even want.
if not, that's totally fine and feel free to reject this.

while working on related features, I noticed the following:

• several structs have a new_with_config() function which takes Option<&config::Config>. these functions then proceed to try and read the config in order to parse the values.
• this is done via an assortment of get_*-or-use-default functions. these all need to handle the case where the config itself is None. what's more, if the config is indeed None, the default is chosen for all of them.
• unit tests all go through this mechanism where the same code paths are used for both "config available" and "config unavailable". in particular, out of 272 tests that go through module_test(), 245 pass None as config which (imho) is just noise.

thus, this PR

1. separates these constructors to either "config is available" new_with_config() which takes &Config instead of Option<&Config>, or "config is unavailable, so use defaults for everything"... in which case I just impl Default::default().
2. changes the get_ functions to also take a &Config instead of Option<&Config>`, which simplifies the logic
3. basic refactors of some of the logic in the areas I've touched.
4. removes some now-unnecessary testing logic which creates dummy data
5. separates module_test() to "with config" and "without config" versions, which removes the None parameter from a whole bunch of existing tests

in the "default" implementations, we can also do something that reuses the default values between the "load from config" and "load defaults" constructors, if you'd like.

[coveralls]

Pull Request Test Coverage Report for Build 15656777377

Details

• 148 of 152 (97.37%) changed or added relevant lines in 14 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 97.566%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/config.rs	9	10	90.0%
src/config/git_config.rs	23	26	88.46%

Files with Coverage Reduction	New Missed Lines	%
src/config.rs	1	87.1%

Totals
Change from base Build 15519682599:	-0.02%
Covered Lines:	4930
Relevant Lines:	5053

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💛 - Coveralls

[MitMaro]

👋🏼 Thanks @pickx for the changes!

I've taken a brief look, I may request some of these be broken out into separate pull requests.

Some of the changes I am very much aligned on, while others less so, so I will try to add some comments around different things.