Add an alternative interface for 'D.C.Install.install'.

Splits 'D.C.Install.install' into three parts:

    * makeInstallContext - load common data
    * makeInstallPlan    - produce the install plan
    * processInstallPlan - actually perform the installations

This allows to manipulate the install plan produced with 'makeInstallPlan'
before performing the installations with 'processInstallPlan'. The high-level
'install' action is still present; most clients should use it instead.