Properly handle workspace inheritance in git dependencies

[ipetkov]

Motivation

Crates from git repos are vendored in a flattened directory where each crate shows up at the root of the vendor directory. Since the vendoring step effectively breaks workspace structures, any crates which use workspace inheritance (e.g. package.version.workspace = true will fail to resolve.

To work around this we inspect the crate's workspace manifest (if it exists) and attempt to manually merge the values while copying the contents to the vendor directory.

Fixes #113

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md