Semantic versions for package dependencies

[donaldoakes]

Package dependencies declared in mdw.yml should support semantic versioning.

The CLI dependencies command should understand semantic versioning when evaluating whether a given dependency is met.

This will also entail a rethinking of the Git auto-import mechanism. Instead of crawling through all branches and tags looking for a matching version, the auto-import should already know which branches/tags to consult. The proposed way to handle this is to specify a list of Git branches/tags to include in the search. This list could be declared in package.yaml or globally in project.yaml.

A complication is the need to authenticate for many Git URLs, while not requiring user credentials or tokens in project.yaml or package.yaml.