Fix relative override paths

Relative paths are now considered relative to the directory containing the
`.cargo/config` file specifying the relative path.

I also merge ConfigValueValue into ConfigValue by moving the Path directly next
to the String that defined it so we can track which strings came from which
paths.

Closes #458
auto merge of #475 : alexcrichton/cargo/issue-458, r=brson

Relative paths are now considered relative to the directory containing the
`.cargo/config` file specifying the relative path.

I also merge ConfigValueValue into ConfigValue by moving the Path directly next
to the String that defined it so we can track which strings came from which
paths.

Closes #458