Make import path replacement hack more reliable

[janopae]

Instead of replacing the string src in the file bin/trelby with the actual location of src on the target platform when installing, we now replace the string "src". This is because sometimes, the replacement seems to happen multiple times, and we want to prevent replacing all occurrences of src in the target path again and again.

This is basically a hack inside a hack, but we want to modernise the installation process anyway (see #75), which might avoid these kinds of hacks in general.

[janopae]

@limburgher does this fix #82?

Sadly, I didn't get to run debuild on my machine and therefore never tested building .deb packages.

[limburgher]

It does not.

[janopae]

Is the resulting bin/trelby file the same as described in #82?

[limburgher]

It is.

[limburgher]

Please close this PR and reopen again https://github.com/trelby/trelby, as the project is merging into that project. Thank you!