`devpy build` should install to a new directory, rather than the build dir

[rgommers]

Installing to the build directory is going to lead to problems (we had those in SciPy already, even if fuzzy on the details right now), that cannot work reliably. There should be a standard install scheme. SciPy does it like this: if the build directory is called mybuilddir, then the install dir is mybuilddir-install.