rust: Resolve crate roots outside rust-project

Previously, we manually re-computed crate roots inside project_json for
rendering rust-project.json. In addition to added complexity, this meant
that generated sources and glob sources would not render correctly - it
would select e.g. `src/**.rs` or `:foo` as a crate root.

Use a centralized computation of crate roots instead.

Bug: 309943184
Test: SOONG_GEN_RUST_PROJECT=1 m nothing, compare rust-project.json
Change-Id: I0caddbf600d025a0041f45e69812cdd6f1761234

rust/proc_macro.go

@@ -78,7 +78,6 @@ func (procMacro *procMacroDecorator) compilerFlags(ctx ModuleContext, flags Flag
 func (procMacro *procMacroDecorator) compile(ctx ModuleContext, flags Flags, deps PathDeps) buildOutput {
 	fileName := procMacro.getStem(ctx) + ctx.toolchain().ProcMacroSuffix()
 	outputFile := android.PathForModuleOut(ctx, fileName)
-
 	srcPath := crateRootPath(ctx, procMacro)
 	ret := TransformSrctoProcMacro(ctx, srcPath, deps, flags, outputFile)
 	procMacro.baseCompiler.unstrippedOutputFile = outputFile