* Restructure Includes3 tests so that I can also test Hackage case.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

* Compute correct install directories for instantiated Backpack components

Previously, we would compute elabInstallDirs once when configuring a
component, and then reuse the exactly same install directories for
every instantiation of the package.  But this is wrong, since we're
installing the header/object files for each instantiation to a different
directory.  We refactor install directory computation into a helper
function and then call it again at instantiation time to refresh
the install directories.

For some reason, this bug ONLY manifests for packages installed from
Hackage; it seems install dirs are not respected for inplace packages.

Fixes #6005

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

* Test fix

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

* Copy backpack version-guard from other backpack testcases