Generate build-globs.ninja when running bp2build.

This fixes null bp2build builds.

When running bp2build, out/soong/.bootstrap/build-globs.ninja wasn't
generated, therefore none of the globs in out/soong/.primary/globs have
actions to generate them. This makes bp2build always re-run on a null
build, because it depends on those non-existent globs in its depfile.

Fix the above by generating a build-globs.ninja file with the globs that
bp2build encounters during conversion, and add tests to verify that null
builds and bp2build -> soong workflows correctly regenerates the
build-globs.ninja file.

Test: soong integration tests for null soong->soong, bp2build->bp2build and bp2build->soong builds.
Fixes: 193650250
Change-Id: Icfbad351dfa85541d1ea451f73e65cac87097926