Cleanup python/ build rules.

Cleanup python/ build rules.
    
    * Remove @ substitutions from python/*.py, making them directly
      importable from source.
    
    * Inline python-module stepmake template
    
    * Install __pycache__ files too; since the Python 3 transition, .pyc
      files weren't installed anymore.
    
    * Move relocate-preamble.py.in to scripts/ . This removes a
      implicit cross-directory (ie. broken) dependency.
    
    * Build python/ first; it's the basis for further scripts

[hahnjo, 2020-03-21 08:56:20]

Could you please copy the comment to all places where you paste the code?

[hanwenn, 2020-03-21 09:47:34]

On 2020/03/21 08:56:20, hahnjo wrote:
> Could you please copy the comment to all places where you paste the code?

sure.

Note that the comment is just paraphrasing the documentation for gettext.install
https://docs.python.org/3/library/gettext.html

[hanwenn, 2020-03-21 11:15:23]

larger cleanup

[hanwenn, 2020-03-21 11:16:12]

PTAL

[hahnjo, 2020-03-21 11:22:15]

On 2020/03/21 11:16:12, hanwenn wrote:
> PTAL

Very please don't clutter diffs with unrelated whitespace changes. If you think
we should apply this, please do so independently. After a message to
lilypond-devel they can probably go into staging -> master directly without much
review.

Also please update the title (here and SF issue) to accommodate to the "larger
cleanup".

[hanwenn, 2020-03-21 11:52:09]

make check passes

[hanwenn, 2020-03-21 12:25:30]

On 2020/03/21 11:22:15, hahnjo wrote:
> On 2020/03/21 11:16:12, hanwenn wrote:
> > PTAL
> 
> Very please don't clutter diffs with unrelated whitespace changes. If you
think
> we should apply this, please do so independently. After a message to
> lilypond-devel they can probably go into staging -> master directly without
much
> review.

For context: many projects that I contribute to keep harping about trailing
whitespace,
hence I added 

  (add-hook 'before-save-hook 'delete-trailing-whitespace)

to my .emacs. I uploaded

  https://codereview.appspot.com/547810043

(I tried doing the whole codebase, but Rietveld times out. Is there any movement
on 
the plan to switch infrastructure?)

> Also please update the title (here and SF issue) to accommodate to the "larger
> cleanup".


Done.

[hanwenn, 2020-03-21 19:32:32]

rebase on whitespace changes

[dan_faithful.be, 2020-03-21 20:27:48]

On Mar 21, 2020, at 08:25, hanwenn@gmail.com wrote:
> 
> For context: many projects that I contribute to keep harping about
> trailing whitespace

I recently started using the ws-butler package to limit whitespace changes to
lines that also have substantive changes.  It has helped me.

https://github.com/lewang/ws-butler
— 
Dan