partial parsing (unsound)

[florianschanda]

This requires #47 (the sound version of this).

Use case:

• provide include directories with new switch (e.g. -I)
• provide one file we're interested in
• validate just this file but ignore global issues

• [ ] determine the set of file (not package) dependencies
• [ ] parse subject file
• [ ] resolve names only for items declared in subject file