Depend on package versions

[richfitz]

I've removed this feature for now, because

• It would require rebuilding after every upgrade of R (because tools, utils, etc will change version number)
• If packages are loaded by a target, the version of those packages is not included in the list (because the version list is determined at maker startup). That then leads to random rebuilds if make is run a second time because now there is a new package loaded! (this interacts with #12).
• Package version information is not granular enough for rapidly changing packages (e.g. if you are developing a package as part of a research problem then you'll want to depend on the code or the git version of that package)

A better interface might be to declare which packages to depend on in different ways (code inspection, number version, git version).

[noamross]

Not sure exactly how this fits in, but maybe documentation could show you how to handle this with packrat.

[richfitz]

Also requested by @aammd here.

[richfitz]

Requested by Will Pearce

could be worthwhile flagging packages to entirely check; that will be useful with tratiecoevo/tree and the scripts directory.