[Distutils] extension to pep 376: "develop" installation

[Chris McDonough]

I propose an extension to PEP 376 to support the development of projects
such that:

- Installation tools should expect to need to cope with files
  present in purelib named "foo.dist-link".  Each of these files
  will be of the form "/path/containing/foo.dist-info\nreldir"
  For example, the contents of such a file might be:

      /home/chrism/projects/zope.interface
      src

  Presence of such files would indicate a link to a directory
  which contains a .dist-info directory for the project as well
  as the subdirectory in the project which actually contains
  packages and modules.

- Installation tools should need to cope with a "develop.pth"
  file in purelib.  Its presence will indicate that there are
  projects in development mode in this environment.
  Its absence indicates that there are no projects in development
  mode in this environment.

- Installer tools should place dist-link files into purelib as the
  result of a notional "develop" command e.g.

      pysetup develop .

  If "pysetup develop ." were run in zope.interface, 
  zope.interface.dist-link would be installed into  purelib

- Installer tools should mutate "develop.pth" as the result of 
  "develop", e.g. "pysetup develop ." would create this
  file if it didn't already exist.

  After a "pysetup develop" of zope.interface it might look like:

  import pkgutil.develop; pkgutil.develop.save_path_order()
  /home/chrism/projects/zope.interface/src
  import pkgutil.develop; pkgutil.develop.reorder_sys_path()

  The "import" lines would ensure that the paths mentioned in the
  .pth file would precede stdlib sys.paths but would come after
  paths mentioned on PYTHONPATH envvar.  Some sort of hack like
  this anyway; maybe Python could offer better support for
  this without hackiness of relying on executing code via
  a .pth file at startup time.

- "pysetup undevelop ." of zope.interface would undo the changes
  made via "pysetup develop"; it would also remove develop.pth
  if no other developed projects existed anymore after 
  zope.interface was undeveloped.

- It will be permissible to have both the "develop" version and
  the "install" version of a library installed in the same purelib.
  Installer tools should be able to cope with this and display
  both versions when they are asked to list installed distributions.

That's the basics.