fix: make proper use of NamedChannelOrUrl

[ruben-arts]

Partly fixes #1764
Fixes #1791

We didn't read the channel name in the mapping correctly anymore. This fixes that.

This now forces the user to have the channel in the mapping defined as an actual channel. Giving the user this error:

❯ pixi install
  × Defined conda-pypi-map channel: bla is missing from the channels, which are: conda-forge, robostack, https://fast.prefix.dev/conda-forge/, bioconda

Added a test to verify

[ruben-arts]

@nichmor The tests are failing but to me it seems like I actually fixed a bug.

pixi/tests/solve_group_tests.rs

Lines 444 to 448 in 1432a0b

	assert_eq!(boltons_first_purl.name(), "boltons");
	assert_eq!(
	boltons_first_purl.qualifiers().get("source").unwrap(),
	PurlSource::ProjectDefinedMapping.as_str()
	);

Here it says that it should take boltons from the ProjectDefinedMapping, but in my new version it tells me it comes from the compressed mapping. To me that seems correct as the custom mapping doesn't have boltons and we don't provide a hash. So it can only come from the compressed_mapping.

Could you tell me whats up

[ruben-arts]

@baszalmstra This reintroduces the channel-config as we're comparing the repodata with the channel thus we require the configuration. Could you rereview?