Allow building the same crate for both machines

[bonzini]

As a first step towards full support for cross compilation of Cargo subprojects, allow building the same crate for both the host and the build machine.

Right now the crate name must match the file name, but that means that the same file name is used for host and build rlibs. To allow that, drop anything after a + sign from the crate name. This convention is already in use, for example Fedora uses it to denote the features that packages support (e.g. rust-$crate+$feature-devel), so it makes sense for Meson to use it instead of reinventing something else.

Other than for cross compilation, this is also useful in some cases where the same crate can be compiled with multiple configurations. Using the "+SUFFIX" syntax avoids having to use a dependency map.

Extracted from #14952.

[xclaesse]

I don't think this is specific to Rust, the same issue happens for libfoo.so compiled for both machines. I'm thinking for example about cross compiling glib where tools (e.g. glib-compile-resources), and thus libs, should be built for both machines.

I know @dcbaker had a much bigger fix to allow building whole subprojects for both machines.

[xclaesse]

I always wanted to be able to do e.g.

host_lib = library('foo', native: 'both')
native_lib = lib.native()

Which is not too different from both_libraries().

[bonzini]

This is true, but then this patch is useful anyway because you need to give different file names to the two build outputs, and map the two file names to the same crate name.

[xclaesse]

Ok, facing the same kind of issue myself, I better understand the logic of this PR. LGTM.