gen_amalgamated: Don't include internal headers in the generated output
gen_amalgamated: Don't include internal headers in the generated output

This patch changes gen_amalgamated to expand .h files included from .cc
files directly in place in the generated .cc file and only adding .h
files that are directly listed in the 'sources' section of the requested
targets into the generated .h file. This avoids accidentally exposing
internal implementation details to the public API users.

Test: tools/gen_amalgamated --build //include/perfetto/public:public //src/public:public //src/public:platform_posix
Change-Id: I7e21160892dc0b28f054eb8a027c6e8e73965e34
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto