Emit a detailed deprecation warning that explains what to do instead.
Also add a unittest.
```
DEPRECATION: target prog links against shared module mymod, which is incorrect.
This will be an error in the future, so please use shared_library() for mymod instead.
If shared_module() was used for mymod because it has references to undefined symbols,
use shared_libary() with `override_options: ['b_lundef=false']` instead.
```
Fixes https://github.com/mesonbuild/meson/issues/9492
It has always been working even if not documented and there is no reason
to not accept it. However, change "True/False" to "true/false" to be
consistent with meson language.
Fixes: #9436
This replaces the absolute hack of using
```
install_subdir('nonexisting', install_dir: 'share')
```
which requires you to make sure you don't accidentally or deliberately
have a completely different directory with the same name in your source
tree that is full of files you don't want installed. It also avoids
splitting the name in two and listing them in the wrong order.
You can also set the install mode of each directory component by listing
them one at a time in order, and in fact create nested structures at
all.
Fixes#1604
Properly fixes#2904
Paolo Bonzini