Leave jamfile modules for user.

[Vladimir Prus]

We currently store a lot of information pertaining to project into
modules for jamfiles. It is better to leave that namespace for users.
This change is a good time to make some cleanups. Subtasks
are:
- Move all the project-related data to the 'project-target' or a new
class.
- Remove 'subproject' rule and introduce 'build-project' rule.
- Introduce 'use-project' rule into project-root module namespace
- Import non-local rules from project-root into project modules
- (Possible) Import non-local rules from parent projects

[Vladimir Prus]

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Implemented the first two parts. Passing over the other two, plus moving 
all data out of project root namespace, to Rene.

[René Rivera]

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Implemented 3 & 4.
Cleanup of project root module space also done.

#5 needs to be decided.

[Vladimir Prus]

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There's no obvious need for #5, so closing this issue.