Allow compiling of non-standalone include files using wrappers

Some include files are not standalone and need to be compiled in context
of another file (called a *wrapper*). This commit allows specifying
these include files and their wrappers in the "include_wrappers" section
in module metadata. YCP Killer then uses this information to execute Y2R
on the wrapper with a special flag (--extract-file), which makes it to
generate Ruby code only for statements from the include file.
README.md: Add info about recognizing non-standalone includes

As suggested by Ladislav Slezák:

  https://github.com/yast/ycp-killer/pull/408/files#r4456156
Merge pull request #408 from yast/include-wrappers

Allow compiling of non-standalone include files using wrappers