bp2build: introduce CodegenMode.

The CodegenMode enum helps to differentiate between bp2build and queryview modes of generating BUILD files from the Soong module graph.

bp2build is used for generating BUILD files from an unconfigured Soong module graph for Soong->Bazel conversion, whereas QueryView is a front-end for querying the existing Soong configured module graph with Bazel query.

Test: go test

Test: TH
Change-Id: I5709f591f1ae08b2770adf272d8ce60c9ee1f46a