Apply performance tweaks to the auto-config path search method.

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This change is in response to your last comment on fabioz#95. When I use the original version of searchPaths with target interpreters on different folders, it didn't seem to have any major problems. Nonetheless, the way the regexs are used allow paths to be discovered multiple times, which cause "absolute path" searches to happen on duplicated items that get discarded anyways. The changes I made here should reduce operations on duplicate items.

I still want to use the regex list as is (ie "python", "python#.#"), though, because the fact that discovered interpreters are often links makes it difficult to sort them by priority after their absolute paths are resolved. Let me know what you think, though.