chore: simplify and cleanup

Signed-off-by: Keith Zantow <keith.zantow@anchore.com>

internal/index/README.md

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+# internal/index
+
+A key-split (radix) index over strings, and a forward/reverse pair of them that answers prefix and
+suffix lookups. Ported from a prototype directory-scan package and kept close to the original so the
+two can be diffed.
+
+## Types
+
+- `KeySplitIndex[T]` / `Node[T]`: a radix tree. Each node holds at most one value and splits its
+  children on the longest common prefix of their keys, so `pom.xml` and `pom.properties` share a
+  `pom.` node. Reads take a read lock per node; writes upgrade to a write lock only where the tree
+  changes. `KeySplitIndex` also marshals to and from a flat JSON object of key → value.
+- `PrefixSuffix[T]`: two `KeySplitIndex` values, one keyed by the string and one by its reverse, so
+  `ByPrefix("pom")` and `BySuffix(".jar")` are both a single descent followed by a collect.
+
+## Operations
+
+| call | answers |
+|---|---|
+| `Get(key)` | the value at exactly `key` |
+| `Set(key, v)` / `Update(key, fn)` | store, or read-modify-write, the value at `key` |
+| `ByPrefix(p)` | every value whose key starts with `p` |
+| `BySuffix(s)` (PrefixSuffix only) | every value whose key ends with `s` |
+
+Values are whatever the caller stores; `internal/archive` stores `[]*node` per base name because a
+name repeats across directories.
+
+## Where it is used
+
+`internal/archive.Index` keeps one `PrefixSuffix[[]*node]` of files by base name. A cataloger glob
+whose last segment is a literal, `*suffix` or `prefix*` (nearly all of them: `**/*.jar`,
+`**/pom.properties`) is answered from it instead of by walking every path.
+
+## Notes
+
+- Lookups are byte-wise; `reverse` works on runes so a multi-byte name reverses correctly.
+- `lock.go` returns unlock functions rather than exposing `Unlock`, which is how a node tells a read
+  lock from a write lock when deciding whether to upgrade.
+- `_find`, `_collect` and `_makeNodeP` are the prototype's internals and are left with its naming.