EA: remove `_TOP_MOD` and just use `Base` (#56581)
The reason we originally used `_TOP_MOD` was to make it possible to load
`EscapeAnalysis.jl` from the `Main` context while developing EA.
However, now that the Compiler stdlib allows the same thing to be done
for the entire `Compiler` module including `EscapeAnalysis`, the trick
on the EA side is no longer necessary.
