coverage: try harder to find the main C source file when the tracer first hits an include file in it

make import of Cython.Coverage module fail if coverage.py isn't compatible (better raise ImportError than false hope)

coverage test: find correct source file for a Cython module in Py3 (which uses platform suffixes for the file names)

--HG--
rename : tests/run/coverage.srctree => tests/run/coverage_api.srctree

coverage: strip CPython platform suffix from shared library file paths when looking for corresponding C file

ensure that we call trace/profile functions without current exception set; reduce code overhead inside of functions for line tracing

fix "debug_trace_code_generation": use truncating instead of just seeking which is not enough to remove an unused marker as it may not get overwritten afterwards