cmake: modules: remove Zephyr module duplicates from ZEPHYR_MODULE_NAMES

This allows users to overrule a module from command line.
As example, the Zephyr build system loads the module FOO from
`modules/FOO`, but user has a custom FOO locally at `custom/FOO` and
would like to use this module instead of manifest specified.

If user does `-DZEPHYR_EXTRA_MODULES=custom/FOO` the following error
will be seen:
```
The binary directory

  build/modules/FOO

is already used to build a source directory.  It cannot be used to build
source directory

  custom/FOO

Specify a unique binary directory name.
```

Removing duplicates from the list allows a user to use
`-DZEPHYR_EXTRA_MODULES=custom/FOO` and thus replace `modules/FOO` for
the current build.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>