[config] Allow external qpOASES dependency

[olivier-roussel]

This PR enables the use of qpOASES from an external package. A similar approach to the one adopted on SOFA is proposed:

• try to find an external package qpOASES first and use it if found
• if not, fallback to the embedded version in code

Thus, this should keep compatibility for current users.

This is part of some necessary steps to clean the code and to go toward a conda-forge package.

[olivier-roussel]

Note that 7593fd8 includes a fix that renames the directory cmake/modules to cmake/Modules, which is the one defined in the CMAKE_MODULE_PATH. So far, all these find modules could not be located on linux due to this case typo (but it seems that none is used actually, excepted the new one for qpOASES).
It should have been in a dedicated commit, I just missed it when squashing... sorry for that

[olivier-roussel]

We'll re-run the CI once #20 is merged and then check for merging