associate measurements with a specific coverage file, not just a raw upload

[matt-codecov]

currently measurements record which raw upload they came from, but a raw upload can contain several separate coverage data files. it would be nice to, for example, tie one measurement to the lcov file and another measurement to the cobertura file

we could accomplish this by replacing raw_upload_id in all of the measurement tables with a coverage_file_id which refers to some new coverage_file table:

create table coverage_file (
    id INTEGER PRIMARY KEY, -- random
    path VARCHAR NOT NULL, -- e.g. "./lcov.info"
    format VARCHAR NOT NULL, -- e.g. "lcov", "cobertura"
    raw_upload_id INTEGER REFERENCES raw_upload(id) NOT NULL
);

existing pyreports will not have this information, so we can just treat them as coming from a single coverage file of format "pyreport"