Handle +build.x properly

[dietmarw]

Currently the versions as seen by impact include the +build.x part not just for listing the available versions (which is fine) but also for installation.

The problems are:

1. When a user does impact install Modelica#3.2.1 nothing is getting installed since impact expects the complete version number including the +build.x part.
2. Dependencies in library only state the version number without any build.

Solution:
When a users does impact install Modelica#3.2.1 simply install the latest available build and the same also when a version number is given as dependency. Still keep the functionality to explicitly state a to be installed build number.

[xogeny]

I consider this part of the work to be done for #83.

[xogeny]

Duplicate of #83.