Make running soong_build in alternate modes nicer.

This includes the JSON graph generator and bp2build.

Before:
GENERATE_BAZEL_FILES=1 m nothing
GENERATE_JSON_MODULE_GRAPH=1 m nothing

Now:
m json-module-graph
m bp2build

They can now also be combined with other targets or each other.

The longer-term goal is to run "m queryview" and "m soong_docs" using
the same infrastructure. There are two alternate approaches:

1. Call soong_build from within the main Ninja invocation. This requires
two sequential soong_build invocations and is thus slower.
2. Do everything requested in the same soong_build invocation. This
would be faster, but one AFAIU can't tell Ninja that multiple possible
actions can build the same output so that doesn't work.

(1) is somewhat more desirable because soong_docs seems to be built
from build/make/core/main.mk ; I assume that that can be worked around
although I haven't checked where the output of "m soong_docs" goes.

Test: Presubmits.
Change-Id: If5ba36490d9f3f60733e6d6be9286eb2b67c3ff5