rust: refactored transformSrctoCrate

Refactor transformSrctoCrate and the functions it calls to not rely
on rust.ModuleContext, preparing it to be callable from soong-cc
context to build a Rust staticlib.

This also refactors out common default flags used when building
libraries, again to prepare for building a Rust staticlib from
a soong-cc context.

Bug: 254469782
Test: m blueprint_tests && m
Change-Id: I678f6fee989c61bb15c340b9887e4d1934991907