build.mcpp: two build-dependencies with the same tail (openmui.rules, gongfu.rules) warn on every build though their qualified names differ #852

What happens

A package has two host-module build-dependencies whose package names share a last segment:

[build-dependencies]
openmui.rules = { path = "../../mcpp/rules", host-module = true }
gongfu.rules  = { path = "../../mgame/gongfu/rules", host-module = true }

Every build of it prints:

warning: build.mcpp: dependency name collides on MCPP_DEP_RULES_DIR (kept '.../mgame/gongfu/rules', ignored '.../mcpp/rules') — rename one dependency to disambiguate

Both rules still work, because each one can ask for itself by its qualified name: mcpp::dep_dir("openmui.rules") gives MCPP_DEP_OPENMUI_RULES_DIR, and the published names include the qualified spelling (#642). But:

• The warning stays on every build, and the fix it suggests (renaming a package) isn't the fix the user needs.
• A rule that asks for its bare name, dep_dir("rules"), silently gets the other package's directory. openmui.rules did this until we changed it to look itself up by its qualified name.

Expected

When two visible dependencies share a bare tail but their qualified names differ, the ambiguous bare name could be left unbound, with no warning: neither package can be the bare name. The qualified names stay unique.

A bare dep_dir("rules") would then answer empty, so a rule that asks for its bare name fails visibly instead of reading the other package's directory. The current warning could become a note that says to ask by the qualified name.

Where

src/build/prepare/graph.cpp fillDepDirs (the published names, including the bound tail) and src/build/build_program.cppm (the collision warning on the MCPP_DEP_<NAME>_DIR and _LINKAGE variables).

Seen in

Sunrisepeak/mui (showcase/fighting's build.mcpp, which uses both openmui.rules and gongfu.rules).