Refactor feature requirements.

[brettviren]

Goals

• have better organization of the "requirements" a feature uses
• automatic documentation of how the configuration parameters feed the "requirements"
• assure consistency when two features use the same requirement
• encourage use of PackageFeatureInfo
• more modularity and reuse of features
• regularize creation of task generators (self.bld()) so common options are consistently passed
• allow for correct expression of inter-step dependencies and for inserting a dependency between two existing ones

  Expected implementation

• Use a single requirements dictionary to hold information on the union of all possible feature requirements.
• This requirements dict maps a configuration key to a namedtuple holding: default value, docstring
• Use a orch_feature decorator which takes a list of required keys and calls the feature_*() method with a PackageFeatureInfo object (PFI)
• The PFI has a bld() method calling self.bld() and assures common args are fulfilled.
• Split up autoconf and cmakemake to configure, cmake and make

[brettviren]

This refactoring mostly went as planned and a lot of things were "touched". The code still deserves a bit more cosmetic refactoring but it's functionality is now much improved.

One deviation from the plan is that I now fully create a package's configuration dictionary, including the contributions from the "feature requirements", before any waf task_gens are created. Mixing in waf too early was the source of a lot of problems and confusions particularly w.r.t. dependencies. Making this separation more clear in the code might be the goal of a future reorganization but for now I think it's okay.

Some small changes at the configuration file level were made. A few key names change (eg, prepare_script changes to prepare_cmd). The "simple", "simple-with-patch" and "art" examples should all be brought up to date and working.

The "features" themselves are no longer written as waf methods but as waf-unaware functions with a decorator to allow for their registration and to pass in their requirements dictionary. They are called with a new version of the "pfi" object (now named "info") and it's through this object that any task construction is done or dependencies are registered or inserted.

When writing each feature's requirements dictionary, one should put the value None unless the feature code knows for sure what a default is. Eg, feature_autoconf "knows" it's prepare_cmd should be the "configure" script but shouldn't make assumptions about the source_unpacked. Also, any key in a feature's `requirementsdictionary must exist in the big list in thefeatures/requirements.pymodule. Likewise, default values in there should beNone`` unless a clear, global default can be decided.

Additional changes is to consolidate the git/hg/svn download features into a single "vcs" one. Likewise, autoconf and cmake features derive from a common "prepare" feature. The common make/make install chunk is now in a "makemake" feature.