[PATCH 2 of 3 V3] demandimport: support "absolute_import" for external libraries (issue4029)

[Jason R. Coombs]

A few comments:

First, the unit test for ambigrel will fail on Python 3. And while Mercurial doesn't support Python 3, I would recommend not adding tests that can be expected to fail on Python 3.

Second, the test appears to include an 'hg id' output which I would expect to change over time. Perhaps the test suite normalizes this so that it's predictable when running the tests, but I wanted to mention it just in case.

Third, the tests seem to be guarded by 'absimport'. I grepped the folder for that indicator but didn't find one. Would this test be run by default? I think it should (though it must be skipped on Python 2.4 as it doesn't support the feature).

Overall, though, +1