Package archives now have priorities.

When installing packages by name, only packages from archives with
the highest priority are considered, before versions are compared.

This solves the "MELPA problem", where MELPA assigns date-based
version numbers to packages which override all other archives.
Giving MELPA a lower priority means packages are installed from
MELPA only when the package is not available from other archives.

This can be overridden manually by the user.