fix: separate golang license caches from mod dir (#2852)

Signed-off-by: Keith Zantow <kzantow@gmail.com>

internal/cache/README.md

@@ -0,0 +1,51 @@
+# Caching
+
+All caches are created from a global `manager`. By defaut this is a `bypassedCache`, which performs no caching.
+One benefit of this is that tests don't need to worry about caching causing issues unless they explicitly need
+to test the cache and can opt-in using the `cache.TestCache(t)` helper.
+
+Syft sets a `filesystemCache` when the [cache options](../../cmd/syft/internal/options/cache.go) are loaded.
+
+When using the `filesystemCache` all items are stored on disk under a root directory, generally in the form of:
+```
+<rootDir>/<named-cache>/<data-version>/path/to/data
+```
+
+# Using the cache
+
+The easiest and preferred method to use the cache is a `cache.Resolver`, which automatically creates a `<data-version>`
+based on the _structure_ of the provided type.
+If the structure changes in any way it will end up with a new version key and all will re populate this new key,
+ignoring cached values from older, different versions.
+The resolver will store items using the `json` package to serialize/deserialize values, so to save space
+it is encouraged  to use `omitempty`. For example:
+
+```go
+type myCacheItem struct {
+	Name string `json:"name",omitempty`
+}
+```
+
+It is possible to use core types such as `pkg.Package` as long as they support the standard `json` serialization,
+but this is discouraged in order to decouple changes to them from affecting the information stored in the cache.
+
+To get a cache for this type:
+```go
+resolver := cache.GetResolver[myCacheItem]("myCacheName", "v1")
+```
+
+Using the `resolver` is a single call, which manages checking for items in the cache, expiry times,
+and if not found invoking the callback to populate the cache and return a value:
+```go
+data := resolver.Resolve("some/cache/key", func() (myCacheItem, error) {
+	// do things to return a myCacheItem or error
+})
+```
+
+If it is common that checking for an item will result in errors, and you do not want to re-run the resolve function
+when errors are encountered, instead of using `GetResolver`, you can use `GetResolverCachingErrors`, which is useful
+for things such as resolving artifacts over a network, where a number of them will not be resolved, and you do not want
+to continue to have the expense of running the network resolution. This should be used when it is acceptable a network
+outage and cached errors is an acceptable risk.
+
+An example can be seen in the [golang cataloger](../../syft/pkg/cataloger/golang/licenses.go) fetching remote licenses.