Python executable path

[weaverba137]

This PR attempts to fix #104 at least for future installs by checking that during the bootstrap process, desiInstall is installed with an explicit desiconda version, not "current".

In theory, it would be possible to reject any attempt to use desiInstall if, internally, it cannot find an explicit desiconda version, but I have not implemented this. I can if that is desired. If we go down this road though, we may want to consider simply eliminating the 'current' symlink entirely, so that it is impossible not to choose an explicit version.

In addition, the desiutil.module file is added to the internal structure of the desiutil Python package so that it is available even if the etc/ directory is not installed, which it won't be, because we no longer copy the full package contents on install.

Finally, there is some clean-up of the package imports in desiutil.install.

[sbailey]

Changes look fine, but I'm not sure how to really test it without impacting our current installations, until we have a new desiconda and can try bootstrapping desiutil into it. OK with me to merge and then make other updates as needed when we are bootstrapping a new desiconda.

[weaverba137]

On edison, I hand-edited desiInstall, and installed desidatamodel 1.2.0. The executable scripts in desidatamodel then had the correct Python paths. So I think we'll be OK with just hand-editing desiInstall on cori & edison for the current desiconda version, then double-checking after the next bootstrap.