Remove 'enabled' properties on builder tools and -p command-line

[lewissbaker]

Now that we have lazy-tasks and named targets we can remove the need for 'enabled' property on tools which was used for switching between different build modes.

For example, specifying -p flag would enable a 'project building' mode which would disable all tools except for the project tool so that you could build .vcproj files without compiling the code.

We should now be able to do this with named targets and lazy tasks/builds instead of disabling certain tools.
eg. have the project tool define an @projects target to allow you to build .vcproj files

[donno]

Would doing so lead to high memory usage as a result of more lambda/captures?
Unlike a normal build, many of these items would never start and thus never be freed.

[lewissbaker]

@donno Yes, that's definitely a possible downside.