Issues 176 and 629: Process extends in order

[distributist]

Process extends in order, otherwise variable settings can be applied out of order.
Fixes #176
Fixes #629.

I first submitted this patch almost 10 years ago. We run a very complex buildout, and this breaks it. I've been patching buildout 2, for almost 10 years, with no issues. The application of this patch to buildout 3 was very straightforward. I believe this fix is stable, and would really appreciate seeing it incorporated.

Thank you.

[icemac]

O no: Ran 616 tests with 15 failures – these are probably failures on master not in your PR.

It seems we need a volunteer to fix the tests on master first.

[distributist]

I spent a couple of hours looking into the tests, and I think it's a CI configuration issue. The tests run fine when I check out buildout and run them locally.

The failing tests follow the same pattern:

File "/home/runner/work/buildout/buildout/src/zc/buildout/tests/repeatable.txt", line 118, in repeatable.txt
Failed example:
    print_(system(buildout), end='')
Expected:
    Getting distribution for 'spam==1'.
    Got spam 1.
    Uninstalling foo.
    Installing foo.
    recipe v1
Got:
    root: root: root: Couldn't retrieve index page for 'spam'
    root: Scanning index of all packages (this may take a while)
    Getting distribution for 'spam==1'.
    Got spam 1.
    Uninstalling foo.
    Installing foo.
    recipe v1

What is causing the test to "fail" is the additional scan for the package(s), as if it's using the wrong location or URL.

I poked around a bit, but unfortunately I don't know Github or the CI system, so I'm not sure where the issue lies. I hope this information helps someone in tracking it down.