getdeps: add build subcommand
getdeps: add build subcommand

Summary:
adds the command that is used to drive a build.

The dependencies of the named project are computed using
the same mechanism behind the `list-deps` subcommand.
If a manifest specifies that it uses the `cmake` builder then
a dependency on `cmake` is synthesized so that we can download
a recent version of cmake ahead of time.

The build command uses the fetcher to update the src and then
executes the builder.

Reviewed By: simpkins

Differential Revision: D14691011

fbshipit-source-id: 31ae59614651ef021a9505e89c13b5717b440071

facebook/wangle,facebook/wangle,facebook/wangle