Prevent import loops

Importing the file with the same name leads to bad loop and consequently OOM.

We should probably record the files being processed (including the main file) and error out in case of loop.

Start at the compiler's import-processing entry point and reproduce an import of a file with the same name. Trace how imported files are tracked, then verify that a repeated file produces an error instead of continuing until out-of-memory; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.