Split resolver.rs into a module (#752)

This is just getting hard to navigate. No code changes, just moving
stuff around.

crates/puffin-resolver/src/resolver/index.rs

@@ -0,0 +1,26 @@
+use url::Url;
+
+use distribution_types::{IndexUrl, PackageId};
+use pep440_rs::VersionSpecifiers;
+use puffin_normalize::PackageName;
+use puffin_traits::OnceMap;
+use pypi_types::{BaseUrl, Metadata21};
+
+use crate::version_map::VersionMap;
+
+/// In-memory index of package metadata.
+#[derive(Default)]
+pub(crate) struct Index {
+    /// A map from package name to the metadata for that package and the index where the metadata
+    /// came from.
+    pub(crate) packages: OnceMap<PackageName, (IndexUrl, BaseUrl, VersionMap)>,
+
+    /// A map from package ID to metadata for that distribution.
+    pub(crate) distributions: OnceMap<PackageId, Metadata21>,
+
+    /// A map from package ID to required Python version.
+    pub(crate) incompatibilities: OnceMap<PackageId, VersionSpecifiers>,
+
+    /// A map from source URL to precise URL.
+    pub(crate) redirects: OnceMap<Url, Url>,
+}