Warn on unreachable_pub - #7762

`doctest = true` on lib target `rustpython_host_env` but source contains no doc tests (set `doctest = false` to avoid running doc tests for this target)

`test = true` on lib target `rustpython_host_env` but source contains no tests (set `test = false` to avoid compiling a test harness for this target)

`doctest = true` on lib target `rustpython_derive_impl` but source contains no doc tests (set `doctest = false` to avoid running doc tests for this target)

`test = true` on lib target `rustpython_derive_impl` but source contains no tests (set `test = false` to avoid compiling a test harness for this target)

`doctest = true` on lib target `rustpython_derive` but source contains no doc tests (set `doctest = false` to avoid running doc tests for this target)

`test = true` on proc-macro target `rustpython_derive` but source contains no tests (set `test = false` to avoid compiling a test harness for this target)

`doctest = true` on lib target `rustpython_compiler_source` but source contains no doc tests (set `doctest = false` to avoid running doc tests for this target)

`test = true` on lib target `rustpython_compiler_source` but source contains no tests (set `test = false` to avoid compiling a test harness for this target)

`doctest = true` on lib target `rustpython_compiler` but source contains no doc tests (set `doctest = false` to avoid running doc tests for this target)