go/build: Build should have an argument to compile one only file

[gopherbot]

by raul.san@sent.com:

build.Build uses all files found using ScanDir. But it's possible that all those files
are not of the same program. I can have a dir. like "/etc/init.d" with
different Go scripts, and each file is a different program.

[rsc]

Comment 1:

You can have such a directory, but you should not.
One directory = one program.

Status changed to WorkingAsIntended.