Extract common CGI and mock code back into VSGI to avoid dependency
between implementations. The CGI base is versioned for the '0.3' major.

Examples do not work yet because we need to set 'rpath' correctly.

Merge FastCGI and SCGI tests in CGI tests because implementations can
only be tested from outside.

This is necessary since they must be loadable from a single location in
the build directory. This should be ensured by '-rpath' linker option.
In the meantime, 'LD_LIBRARY_PATH' can be exported:

    LD_LIBRARY_PATH=src/vsgi/servers examples/app/app

Remove pkg-config and deps files.

…arch path

Examples work and subprojects should work as well.

This allow one to obtain multiple instances of different server
implementations.

Fix the FastCGI server tests since it calls 'Server.@new' twice with the
same implementation.

Using '$ORIGIN' makes it possible to refer to relative directory for
searching server implementations.

After the installation, the '-rpath' is set to '$ORIGIN/vsgi/servers' so
that it reflects the system-wide folder.

Replace previous usages with 'Server.new_with_application' which
initialize the instance with an initial application callback.

Ensure that basic implementations exist and can be successfully loaded.