In order to keep "import umodule" working, the existing mechanism is
replaced with a simple fallback to drop the "u".
This makes importing of built-ins no longer touch the filesystem, which
makes a typical built-in import take ~0.15ms rather than 3-5ms.
(Weak links were added in c14a81662c)
This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
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