More robust scheme for locating imported and included .piqi modules (see #43)

When Piqi looks for included or imported `.piqi` module by its module name, it
tris one or more directories until it finds the matching file. The lookup order
is defined as follows:

- directory from which the referring module was loaded
- search paths explicitly provided by using the `-I ..` command-line parameter,
  which can be specified more than once
- current working directory (`.`)
- search paths listed in the `PIQI_PATH` environment variable, with several
  paths separated by `:`.