functions/package: change values for PKG_VERSION_xxx constants

[jouvin]

Current values are rather misleading/counter-intuitive. And they don't allow to do a numeric comarison like:

if ( pkg_compare_version('x', '1.6.0') <= PKG_VERSION_EQUAL ) ...

to test if a version is less or equal.

This should not be a problem to define less=-1 and greater=1 when all the PRs related to pkg_compare_version() have been merged. We just need to tell site to update their local templates if they use this function.

[jrha]

Assign a new milestone if and when a PR exists.