BUILD_DEPENDS and external pkgsrc tools

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

Aleksey Cheusov just alerted me to a problem that my recent changes
for cross-compilation causes:  External tools operating on pkgsrc may
want to examine BOOTSTRAP_DEPENDS, BUILD_DEPENDS, and DEPENDS in order
to find all dependencies, but now there's an extra TOOL_DEPENDS which
they have to know about.

He suggested that for native builds, we should just move everything in
TOOL_DEPENDS to BUILD_DEPENDS to avoid breaking these tools.  I'm not
sure yet where this would go.

Thoughts?  How stable are these pkgsrc variables/internals supposed to
be?