execute "clean" by default after new catalog download?

[jrmarino]

It's common for dependencies of a package to change without the version of the package to change.
It seems like during the upgrade phase, rvn sees a package with the same version (although different checksum) and rvn just moves on.

By having clean remove cached packages where checksum has changed, rvn should download and re-install the package.

seen by:
neumann

[jrmarino]

design new test case:

test case:

1. build and package a shared library
2. another package that depends on first
3. genrepo
4. install second package (pulls in first)
5. build and package update to shared library (new LIB PROVIDES)
6. rebuild second package (should have different LIB REQUIRES), no version bump
7. genrepo
8. upgrade all (should attempt to upgrade both packages)