Get import path from go.mod

[first-vil]

Overview

I fixed #33

Changed points

• In main.go, There was a method findInModule that gets relative path from go.mod . I transformed it to a function to reuse it
• Implemented findPackagePath to calculate the import path, and replaced existing process with it

Confirmation

I confirmed it works on a project structure shown as examples in #33

% gobco # existing version 1.3.2
# github.com/1st-vil/awesomeProject/something_test [github.com/1st-vil/awesomeProject/something.test]
./gobco_bridge_test.go:6:19: undefined: something.GobcoCover
FAIL    github.com/1st-vil/awesomeProject/something [build failed]
FAIL
go test .: exit status 1

% ~/gobco/gobco # executable file created from this PR
ok      github.com/1st-vil/awesomeProject/something     0.457s

Condition coverage: 0/0

[dakusui]

Hi!
@rillig (Cc: @junhwi @Pezzah )

Actually, I really want this PR to be merged as my team is experiencing the same problem as #33, where C1 (branch) coverage can't be measured for packages (xyz) that contain a test whose package is xyz_test.
Please let me know if anything that I can help.

Thanks a lot in advance!

FYI: In case you think closed conversation is needed, I sent out a linked in connection invitation. Please accept it and let me know your thought.