Allow symlinks to be included while collecting exported headers.

Bug: 62380238

Also:
1) Some code clean up to avoid copying set of exported headers.

Test: The following:
      1) change build/soong/cc/builder.go to use built tool instead of prebuilt.
      2) make liblog/include/log.h a symlink to a moved copy -> liblog/log.h.
      3) mm -j64 in liblog, liblog.so.lsdump has enum android_LogPriority, which
         is not present without this change.

Change-Id: Iaecfad5b638ee0db8343bbd80eb60c6aa8f1feac