[GR-51173] Improve computation of suite root dirs.

[ansalond]

To locate the root dir of a suite, mx:

• checks if the suite is contained in one or more multiple VCS, returning the one with the deepest nesting
• if no VCS is found, looks at .mx_vcs_root and ci.hocon files

We should try both approaches and return the "most nested" result (that is, the longest path, which we assume it's the closest to the suite).

[ansalond]

/cc @zakkak, since these changes affect #221

[zakkak]

LGTM, thanks @ansalond