Export OUT_DIR variable to rust-project.json

This variable is required by rust-analyzer to correctly process crates
that uses the include!(concat!(env!("OUT_DIR"), ...)) pattern.

Adds an initialize method to baseCompiler to save the computed path for
this directory. It is not possible to use the BeginMutator as the
BaseModuleContext does not contain enough information to use
PathForModuleOut.

Bug: 175004835
Test: SOONG_GEN_RUST_PROJECT=1 m nothing; inspect rust-project.json
Change-Id: If47b3832d3cca5712ae87773c174a61f5ee27bf8