Fix #2137: Create dummy companions for top-level objects without a real one

Previously, we sometimes ended up forcing a companion class symbol from
a previous run or from the classpath which lead to weird issues like in
`false-companion`. Even if we end up not forcing such a symbol, its
presence can still lead to issue: before this commit incremental
compilation of `dotty-compiler-bootstrapped` was broken because we
recorded a false dependency on the non-bootstrapped `dotty-compiler`
jar.

The added test is currently marked pending because it does not work with
JUnit (which doesn't handle separate compilation), only partest. I
didn't managed to get it to work right, and this won't be necessary once
our testing framework is overhauled by
https://github.com/lampepfl/dotty/pull/2125 anyway, so I'll just have to
remember to enable this test afterwards.

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