import-blocks: Do not read `stdin` to memory (#11072)

* import-blocks: Do not read `stdin` to memory

This fixes a bug with `import-blocks` reading the entire `stdin` before starting to import the
blocks. However, for huge files that uses quite a lot of memory. We can just read from `stdin` step
by step as we do it with a file. This ensures that we don't read the entire input at once into memory.

* FMT

* Fix warning