extensions: move the "import" logic out from "load" The "load" method does too many things: on-demand import and check version. This patch moves the import logic out from "load" so it could be wrapped to change the import behavior, for example, chg will use it to pre-import extensions.

hgmanpage: stop using raw-unicode strings These don't exist in Python 3, and this ends up looking a little more explicit to Martijn and me anyway.

revset: define _symletters in terms of _syminitletters