Partial bp2build conversion of bootstratp_go_package

This module type does not implement android.Module, and therefore we
cannot write a conventional bp2build converter for this module type.
Instead this has been special-cased inside bp2build/build_conversion.go.

Because of the above, we also do not have access to useful functions
available in the ctx object of ConvertWithBp2build. This includes
1. Finding the package (directory) of a dep. This requires getting a
   handle of the underlying module from its name (string). To solve, we
   do a pre-visit to collect this information. This did not increase the
   wall time. On my machine, `m bp2build --skip-soong-tests` takes ~14s
   before and after this CL
2. Converting srcs to labels. This requires glob and package boundary
   resolution. This CL introduces a partial implementation for this
   function. (glob patterns are not used in go tools)

For (1), I considered creating a `ModuleFromName` on
`blueprint.Context` instead of a pre-run, but this increased the time to ~27s.

Test: unit tests
Test: TH
Bug: 284483729

Change-Id: Ifeb029103d14947352556dba295591dd7038b090