move prepare_step before extract_step

[boegel]

cfr. #1376

I don't actually intend to get this merged, I just want to get this on record...

I started looking into this, and it quickly became clear that @wpoely86's view was right: moving up the prepare_step such that dependencies required for unpacking sources is quite intrusive to do.

The guess_start_dir() call had to be moved from prepare_step to the end of extract_step, since it obviously requires that the sources are unpacked. That's a fairly minor issue, more like a logical consequence (you could even argue that guess_start_dir() is out of place in prepare_step, I guess).

As anticipated in #1376, this impacts easyblocks where the prepare_step is customized and it is assumed that the sources are already unpacked when it is being called. At first sight, this affects at least two software-specific easyblocks in the central repository: DL_POLY_Classic and OpenCV.
Although it's fairly easy to fix them to take into account that prepare_step is run before extract_step, it highlights the impact that the change has (we should take into account that there are other (customized) easyblocks outside of the central repository that may be affected).

This change also breaks a test (test_guess_start_dir), which may be easy to fix (it looks like a problem with assumptions made by the test itself, rather than an actual problem being caught), but it's still a sign of trouble...

[boegel]

Closing this, since I don't actually intend to get this merged...