internal/pkg: compute identifier from deps

This provides infrastructure for computing a deterministic identifier based on current artifact kind, opaque parameters data, and optional dependency kind and identifiers.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/net.go

@@ -53,6 +53,9 @@ func (c *Cache) NewHTTPGet(hc *http.Client, url string, checksum Checksum) (File
 	return c.NewHTTP(hc, req, checksum), nil
 }
 
+// Kind returns the hardcoded [Kind] constant.
+func (a *httpArtifact) Kind() Kind { return KindHTTP }
+
 // ID returns the caller-supplied hash of the response body.
 func (a *httpArtifact) ID() ID { return a.id }