[import] implement file lookup
[import] add single file importing routine
[import] implement automated batch import

Already being used for the next update commit.
[import] prefer enumerations instead of strings
[import] classify essential and non-essential files into appropriate subdirectories

Simplifies writing make rules ;)
[build] update import path in makefile

As a consequence, imported files not needed at runtime will no longer be
packaged.
[import] fix error message
[import] fix destination path building in import procedure

D’oh!
[import] unify appearance
[fontloader] reorganize under the new import scheme

For better orientation, the fontloader tree now contains two
subdirectories into which files have been reorganized:

The two files required at runtime for the fontloader and luaotfload-tool
are:

      × ``fontloader-basics-gen.lua``
      × ``fontloader-fontloader.lua``

They are now kept in the ``src/fontloader/runtime/`` subdirectory.

All other files from upstream are now located in
``src/fontloader/misc``. This includes a number of files that have not
yet been part of Luaotfload. Currently, the *misc* set of files is not
packaged along with Luaotfload. This may change in the future when there
is an option to switch the merged fontloader for its constituent files,
or even for upstream.