Replace target attribute with targets

[attilaolah]

This is so that multiple labels could be provided to a toolchain as sources. An example is emcmake, which needs both emscripten and binaryen to run.

Another reason for this is to pass in config files that might be needed for the toolchain. The config files can be passed in as a label and will be available in ${EXT_BUILD_DEPS}/bin, just like any other directory.

[attilaolah]

Actually, let's hold off with this: it should be possible to use a file group or something similar to get this to work.

[jsharpe]

Actually, let's hold off with this: it should be possible to use a file group or something similar to get this to work.

Yes that's what I was also thinking; the other way is a sh_binary wrapper script which sets up the PATH env before calling out to the target to reference the tools stored in a data attribute on the sh_binary?

Also see the changes I'm making in #437 - enabling use of $(execpath) in the path attribute - does that help?

[attilaolah]

Yes, let me revert this back to a draft PR while I try to plumb something together on my side.

[attilaolah]

I'm trying to get this to work, but I think it won't be possible until #437 is merged.

• If I pass in a filegroup as the target label, none of the sources of the filegroup are even built.
• If I pass in a sh_binary rule, it seems to work but then I lose all the runfiles I still don't get the data files of the corresponding sh_binary built for the host.

[attilaolah]

I finally managed to work around this, but I basically had to inject all required tools into tools_deps, which is annoying a bit but I have macros for each rule anyway.

It's not great but should work until #437 is done, and so I don't need this change any more.