Create a symlink to the built Python binary modules so that they can be
imported as `Package.Module` (same as it is the case once they are
installed).

This is a follow up of !1354. There PkgConfig is only looked up if
the `pkg_check_modules` macro hasn't been define. The problem is that
the scope of macros seem to be global, and finding PkgConfig in another
directory defines that macro. However, variable scope is per directory
and the cmake of PkgConfig doesn't really work.

This changes the check to use the `PKG_CONFIG_FOUND` variable

so that things work in a super project build

This allows for super project builds to have separate merge
targets for every project.

This was discovered thanks to a name collision between Eigen and DD4hep,
Eigen having an header file named "Core" (with no extension indeed) and
DD4hep a directory with same name. DD4hep happened to come first in the
dependency list and thus the directort was used...

Note that this reveal a deeper problem in the scanning made by this tool
: no scoping and thus no cleaning of the list of directories when going
to next file. This is the actual root of the problem that lead to DD4hep
being first in the dependency list

- fix runtime path precedence order
- allow clients to override the way runtime paths are managed

as they have their internal timeout check

The test corresponding to `gaudi.qms/qmt_args.qmt` was wrongly named
`gaudi_args` because of the order of string replacements and the filname
prefix `qmt`.

For a nice introduction to `FIXTURES_*` in CTest see
[here](https://crascit.com/2016/10/18/test-fixtures-with-cmake-ctest/).

`FIXTURES_REQUIRED` is set in addition to setting `DEPENDS` for ever test that
defines `prerequisites`. Using `FIXTURES_SETUP` a fixture is defined for every
test that is a dependency of another, where the fixture name is identical to
the test name.

The main advantage of using fixtures is usability: `ctest -R <regex>`
will also run the dependencies, unless explicitly ignored (e.g. with `-FA .*`).
Moreover, when a dependency (fixture) fails, the dependent test is skipped
("Not Run").

it was actually emptying the link list if
GAUDI_PREFER_LOCAL_TARGETS was TRUE

At build time the requested optional dependencies are "REQUIRED", while
they are just RECOMMENDED when in GaudiConfig.cmake is used in
downstream projects.