feat: support dotted dependency selectors

[Sunrisepeak]

Summary

• add a shared dotted dependency selector model with ordered candidates
• support omitted-mcpplibs priority in root/dev/build/workspace deps and xpkg.lua mcpp.deps
• preserve dotted selectors in mcpp add while keeping ns:name as explicit namespace syntax
• bump pending release version to 0.0.43 and document the change

Selector Rule

Single-table dotted selectors are ordered, not forced under one namespace:

• imgui.core tries mcpplibs.imgui/core, then imgui/core
• imgui.backend.glfw_opengl3 tries mcpplibs.imgui.backend/glfw_opengl3, then imgui.backend/glfw_opengl3
• explicit subtables such as [dependencies.compat] gtest = "..." remain direct compat/gtest

Verification

• mcpp build
• mcpp test
• MCPP=target/.../bin/mcpp bash tests/e2e/12_add_command.sh
• MCPP=target/.../bin/mcpp bash tests/e2e/62_dotted_dependency_selector_priority.sh
• MCPP=target/.../bin/mcpp bash tests/e2e/52_local_path_namespaced_index.sh
• MCPP=target/.../bin/mcpp bash tests/e2e/58_preinstall_mcpp_deps_for_hooks.sh