Wrong module name for `-source` mode.

[python-processing-unit]

According to the specification section 12:

• INT: MAIN() - Returns 1 when the call site belongs to the primary program file (the file passed as the interpreter's first argument, or <string> when -source is used). Returns 0 when executed from code that came from an IMPORT (including nested imports). The result is determined solely by the source file that contains the call expression, not by the caller's call stack.

In the implementation, however, the module name used when the interpreter is run in -source mode is <source>, not <string>.

& ".\prefix.exe" -source "ASSERT(0d0)"
Traceback (most recent call last):
  File "<source>", line 1, in <top-level>
    ASSERT(0d0)
    State log index: 0  State id: s_000000
RuntimeError: Assertion failed (rewrite: EXPR)