feat: reorganize packages

pkg/command/command_errors.go

@@ -0,0 +1,21 @@
+package command
+
+import "fmt"
+
+// ErrRequiresLeadingSlash is returned when a parsing a string that
+// doesn't start with a [leading slash character].
+//
+// [leading slash character]: https://github.com/ucan-wg/spec#segment-structure
+var ErrRequiresLeadingSlash = fmt.Errorf("a command requires a leading slash character")
+
+// ErrDisallowsTrailingSlash is returned when parsing a string that [ends
+// with a trailing slash character].
+//
+// [ends with a trailing slash character]: https://github.com/ucan-wg/spec#segment-structure
+var ErrDisallowsTrailingSlash = fmt.Errorf("a command must not include a trailing slash")
+
+// ErrRequiresLowercase is returned if a Command contains, or would contain,
+// [uppercase unicode characters].
+//
+// [uppercase unicode characters]: https://github.com/ucan-wg/spec#segment-structure
+var ErrRequiresLowercase = fmt.Errorf("UCAN path segments must must not contain upper-case characters")