emit_headers - controls whether header files are included in the generated compile_commands.json
emit_externals - same, but for external sources or headers - bazel external workspace sources, etc.

Both default to True - the existing behaviour.

Some tools like ccls don't work well with headers in the compile_commands.json, and some large
projects can generate very large compile_commands.json databases when including all external files.

Rename attributes - start with exclude_
Rework external exclusion algorithm - use action targets and look for  prefix
Tweak how python script passes options around

Remove extra whitespace
Tweak  macro - default args are set to None, rely on inner rule's attribute's default values

Changed attributes to exclude_external_sources and exclude_headers

Change exclude_headers options to just all and external
Optimize some header eclusion cases (thanks @cpsauer)
Update docs

Moved missing file error check to _get_files, names changed

Also improve assert messaging and remove dead assert.

Plus, fix a misplaced comment and some typos, old and new

And do a little associated cleanup

… typo

…ters

Sad, but probably necessary to keep most folks moving forward

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