Let a project override the PyPI to conda name map

[timkpaine]

The vendored PyPI→conda table only covers what conda-forge knows about. A project depending on distributions packaged elsewhere — or packaging its own — had no way to say how those names translate, short of waiting for the table to be regenerated from grayskull.

Adds two optional tables to the [tool.nepenthe] stanza:

[tool.nepenthe.package-mappings]
example-package = "example-conda-package"

[tool.nepenthe.extras-mappings]
"example-package[extra]" = ["example-package", "example-package-extra"]

package-mappings renames a distribution, taking precedence over the vendored entry (or supplying one where there is none).

extras-mappings covers a case a rename cannot express. conda has no equivalent of a PyPI extras group, so a distribution that splits its optional features into separate conda packages is a one-to-many translation. parse_requirement previously discarded the extras group outright, which made this invisible.

Behaviour

• check requires every package an extras group expands to, reporting the first that is missing or in conflict.
• try adds one conda spec per package, each carrying the requirement's specifier.
• An extras group with no mapping still resolves to the distribution name alone — the same result as before, so nothing changes for existing projects.
• Keys are normalized, so casing, separator style and extras order are irrelevant. An extras-mappings key with no […] group is ignored.
• try --project reads both tables without requiring the rest of the stanza.

API

check_dependencies and requirements_to_conda_specs take a &name_map::Overrides; parse_requirement returns the extras group. New: name_map::Overrides, name_map::extras_key, ProjectRef::name_overrides, project::read_name_overrides. The Python surface is unchanged.

[github-actions]

Test Results

189 tests  +7   189 ✅ +7   8s ⏱️ +2s
  5 suites ±0     0 💤 ±0 
  2 files   ±0     0 ❌ ±0 

Results for commit 0925860. ± Comparison against base commit d5202b1.

This pull request removes 1 and adds 8 tests. Note that renamed tests count towards both.

nepenthe-core ‑ project::tests::parse_requirement_extracts_name_and_specifier

nepenthe-core ‑ name_map::tests::a_package_override_wins_over_the_vendored_table
nepenthe-core ‑ name_map::tests::an_extras_override_expands_to_several_packages
nepenthe-core ‑ name_map::tests::an_unmapped_extras_group_resolves_to_the_name_alone
nepenthe-core ‑ name_map::tests::extras_keys_without_a_group_are_ignored
nepenthe-core ‑ name_map::tests::overrides_default_to_the_vendored_table
nepenthe-core ‑ project::tests::check_expands_an_extras_group_and_requires_every_package
nepenthe-core ‑ project::tests::check_prefers_a_package_override_over_the_vendored_table
nepenthe-core ‑ project::tests::parse_requirement_extracts_name_extras_and_specifier