[bldr-build] Add `pkg_build_deps` and `PATH` fixes.
[bldr-build] Add `pkg_build_deps` and `PATH` fixes.

Build Dependencies
------------------

This change introduces a new Plan variable: `pkg_build_deps`, also known
as build dependencies. This is an array, similar to `pkg_deps`
containing a list of packages needed to **build** the plan, but not to
**run** the resulting package. For example, a Redis plan may have a
build dependency on gcc and make.

The dependency metadata is added to the package metadata and a new file
`BUILD_DEPS` is also created. This gives bldr packages enough metadata
to answer the following questions (previously difficult or impossible to
determine):

* What verion of gcc was I build with?
* What version of musl was statically compiled into me?
* What version of Go built this service?

Remove gpg-zip Dependency
-------------------------

Previously, a wrapping script `gpg-zip` was used to tar and gpg sign the
resulting bldr package. This has been replaced with a comparable
`tar ... | gpg ...` form which now only requires the `gpg` binary being
present in the build environment. A future version of the build
environment may add a statically built `gpg` binary to simplify the
build dependency chain of `bldr-build` itself.

Build-Time PATH Calculation
---------------------------

This change prepends its own `pkg_binary_path` and all dependant build
and runtime dependencies' `pkg_binary_path` to the current `PATH` before
the build callbacks are invoked. The build dependencies' `PATH` entries
will be added before the runtime dependencies, which is either the
correct behavior or a future technical flaw (time will tell here).

nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,georgemarshall/habitat