This PR adds a module resolution procedure to Lake to disambiguate
modules that are defined in multiple packages.
On an `import`, Lake will now check if multiple packages within the
workspace define the module. If so, it will verify that modules have
sufficiently similar definitions (i.e., artifacts with the same content
hashes). If not, Lake will report an error.
This verification is currently only done for direct imports. Transitive
imports are not checked for consistency. An overhaul of transitive
imports will come later.
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