Allow project overrides via CMTPROJECTPATH

[garonne <garonne@…>]

It would be useful to be able to override a project version using CMTPROJECTPATH in the same way as a package version can be overridden in CMPATH. This would facilitate testing of new versions of projects by putting the new version (e.g. AtlasCore/2.3.0) in a location referenced by an earlier CMTPROJECTPATH component.

[garonne]

If you use wilcard for project dependencies (see ticket #29) , this is actually true.

Let consider the the project A use project B with the following settings:

> ls
<project_area>/A/cmt 
<project_area>/A/cmt/project.cmt
...
<project_area>/B/v1/cmt
<project_area>/B/v1/cmt/project.cmt
...
<project_area>/Ext/B/v2/cmt
<project_area>/Ext/B/v2/cmt/project.cmt
...
>echo $CMTPROJECTPATH
<project_area>/Ext:<project_area>

By specifing use v* within A/cmt/project.cmt, we obtain:

> cmt show projects
...
  B (in <project_area>/Ext/B/v2) P=...
...

By specifing use v1.* within A/cmt/project.cmt, we obtain:

> cmt show projects
...
  B (in <project_area>/B/v1) P=...
...

So i think we can consider this one is closed.