perf(io): refactor to not read line beyond the end of "import"

[JohelEGP]

Implements #515 (comment).
Not a big deal, just optimizes the case where a line starts with a long identifier.

100% tests passed, 0 tests failed out of 668

Total Test time (real) =  92.99 sec

clang-12-libstdc++-e.cpp2... ok (all Cpp1)

real 0.38
user 0.37
sys 0.01
gcc-10-libstdc++-e.cpp2... ok (all Cpp1)

real 0.39
user 0.37
sys 0.01
msvc-msstl-e.cpp2... ok (all Cpp1)

real 0.53
user 0.52
sys 0.01