gnulib-tool.py: Use a standard Python program directory structure

[Bruno Haible]

Here is a patch that makes gnulib-tool.py use the standard Python program
directory structure:
  - a single .py file in the parent directory,
  - any number of .py files in a subdirectory.

With this, no PYTHONPATH override is needed any more, and the import-related
error markers in Eclipse are gone.

Since this change has consequences on the Eclipse project configuration, I
incorporated the instructions from
<https://lists.gnu.org/archive/html/bug-gnulib/2024-03/msg00157.html>,
with the necessary changes, into the HACKING file. Collin, feel free
to document the way you install, configure, and customize PyCharm in
the same file.

Bruno

0001-gnulib-tool.py-Use-a-standard-Python-program-directo.patch
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0002-gnulib-tool.py-Add-developer-documentation.patch
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