feat(nest): update nest extension installation dir

[sanjayankur31]

The /nest bit seems to be included in NEST_LIBDIR now, so does not need to be explicitly added:

https://github.com/nest/nest-extension-module/blob/master/CMakeLists.txt#L178

I expect nest-config --libdir provides the complete path already.

[coveralls]

coverage: 71.994% (+0.009%) from 71.985% when pulling c6de29f on sanjayankur31:feat/nest-install-dir into 75cacd6 on NeuralEnsemble:master.

[apdavison]

@sanjayankur31 for me nest-config --libdir gives just "lib". This is for installing into a Python venv (-DCMAKE_INSTALL_PREFIX=$VIRTUAL_ENV).

Do you see something different?

[sanjayankur31]

I haven't tested just in a virtualenv. This is for the Fedora build where we explicitly set the CMAKE variables etc.. We do set -DCMAKE_INSTALL_LIBDIR:PATH=/usr/lib64. So, the shared objects end up into %{_prefix}/{lib,lib4} without a /nest suffix:

$ nest-config --prefix
/usr

$ nest-config --libdir
/usr/lib64

$ rpmls nest | grep lib
..
lrwxrwxrwx  /usr/lib64/libnest.so.3
-rwxr-xr-x  /usr/lib64/libnest.so.3.7
lrwxrwxrwx  /usr/lib64/libsli.so.3
-rwxr-xr-x  /usr/lib64/libsli.so.3.7
lrwxrwxrwx  /usr/lib64/libsli_readline.so.3
-rwxr-xr-x  /usr/lib64/libsli_readline.so.3.7

I do see that in the latest code for the nest-extension-module, they don't use the /nest/ suffix either:
https://github.com/nest/nest-extension-module/blob/7273612bef402838df69a13f46f9f9be09fd9ea9/CMakeLists.txt#L147

but maybe this patch isn't really required upstream (and we just carry it downstream so that pynn matches what we do with nest)?