better merging of test project file

[00vareladavid]

The current test project merging has two flaws:

1. Detecting whether two dependencies are actually the same. It uses dumb string comparison without any normalization.
2. It errors out when it detects two conflicting deps. This is just unnecessary.

This PR fixes both of those:

1. Determine the actual location we expect to load the source from and more intelligently determine if these two paths are the same (this is samefile in the code)
2. In the case where to deps conflicting, we should just warn, choose the dep specified by the main project, and move on (basically what @fredrikekre recommended in "cannot merge projects" error #1585 )

Additional note:
samefile should ideally just compare the filesystem node id, but the dep might not exist at this point, so we have to fallback to string comparisons in the worst case.

fix #1585

[tkf]

2. choose the dep specified by the main project, and move on

This is not always the best choice, as discussed in #1585 and #1714.

[schlichtanders]

If there is no agreement on 2., couldn't we just have 1. for now?

It would already solve some setups, like e.g. dev ..