Refactor file path shortcuts

[seaneagan]

Do we need the following?

WEB_DIR, LIB_DIR, BUILD_DIR, BIN_DIR, dartVM.

The names all violate the style guide, which would suggest:

webDir, libDir, buildDir, binDir, dartVm

The dirs could make sense as part of a class which represents a pub package:

class Package {
  Directory get lib => new Directory('lib');
  Directory get bin => new Directory('bin');
  Directory get test => new Directory('test');
  Directory get web => new Directory('web');
  Directory get build => new Directory('build');
  // Anything else here?
}

Package package => new Package();

dartVm could make sense as part of #204:

vm.path

[devoncarew]

They're used as shorthand to access common project locations in build scripts. The pre-date the style guide change. They could be deprecated and replaced with webDir, libDir, buildDir, binDir, dartVm. They don't represent any general package, but the paths for the current one (the one the build script is running in). So I'd say we should keep accessing them terse (just buildDir, not currentPackage.buildDir).

[seaneagan]

top-level webDir etc. sgtm. I think dartVM will probably no longer be needed after #204 is resolved.

[seaneagan]

Do we want to add testDir, toolDir then?