Add support for foreign-library to v2-install

[matil019]

cabal v2-install . does install foreign libraries (foreign-library) into the store but doesn't symlink/copy them into ~/.cabal/lib directory.

3.3.2.8.Foreign libraries, Cabal User's Guide says

... we install foreign libraries in ~/.cabal/lib, much like we install executables in ~/.cabal/bin.

but it only applies to v1-install.

I'd like to v2-install foreign libraries, not just v2-build, because the former embeds proper rpaths that refer to ~/.cabal/store/, whereas the latter leaves references to dist-newstyle/.

Currently I'm doing something like:

find ~/.cabal/store -name 'libmyforeignlib.so' -exec ls -t {} + |
head -n1 |
xargs cp -t /path/to/lib/

but I'd like to be able to do something like this instead:

cabal v2-install --installdir-lib=/path/to/lib/ .

[23Skidoo]

/cc @edsko @typedrat