Enforce rule that first path in [paths] must exist, and choose paths based on that.

[nedbat]

The [paths] docs say:

The first value must be an actual file path on the machine where the reporting will happen, so that source code can be found.

But there is no immediate error if the first value is not a path. The existence of the path should be confirmed.

Further: if there are two [paths] sections, and a file path matches two of them, the one whose first entry exists should be chosen.

[nedbat]

/cc @jmbowman

[nedbat]

This is improved in 7e7c44b. Now a remapped path must exist, which means the prefix must exist. If there are multiple alias mappings, the one that finds an existing file will be used.

[nedbat]

This is now released as part of coverage 7.0.0b1.