Make import alias generation predictable

[rmohr]

Instead of iterating over a set and generating aliases for conflicting
immports, iterate over a sorted map based on this set. This ensured
predictable output and makes automated checks possible.

We for instance had the problem that our automated check if go generate was run, sometimes fails because of different generated code by gomock.

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[rmohr]

I signed it!

[fabiand]

@googlebot @rmohr is covered under the Red Hat CLA.

[eparis]

@googlebot he signed it!

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