Move cquery RequestType interface to bazel_handler

go idiom is to define the interface where it is used rather than where
it is defined. This makes it obvious that ParseResult is not a used part
of the interface, removing the need to return an interface{} and cast
results.

Test: go test soong tests
Test: generate & sync bp2build; mixed build libc
Change-Id: I0d8d99c1d8d0125588522cc86502286b83c91bf7