Replacements should allow for version mismatch

[vvuk]

Right now, if I'm developing updates to package A that package B depends on, if I use the replacements mechanism, I cannot bump the version number in package A -- even if the work I'm doing is intertwined, and I'm not going to bump the version that B requires until A is ready. For example, I may have introduced a new api in A that I want to use in B -- but B's usage is uncovering issues in A, that need patching (after the original PR would have already been accepted in A).

Replacements would be far more useful if I could specify "foo:0.1" = { git="..." } and have this mean "assume whatever is in the git dir is 0.1, even if it indicates otherwise in a version. A warning can get printed (e.g. "Warning: replacement create 'foo' has version 0.2, but it's replacing version 0.1") so that when this isn't desirable it's easily caught.

[alexcrichton]

I believe this is a dupe of #2649, but thanks for the report!