`Update()` gets repo names before resolving a lock file by calling
`resolveRepoNames(req)`. But that method changes aliased repo URLs into
the actual URLs. That makes digests from `helm update` and `helm build`
be different for each other.

To make them in sync, setting actual (resolved) repo URLs into the
loaded chart during `helm build` is necessary. Thus, this commit adds an
extra step in the `Build()` implementation.

For comments, this commit also changes the name of `getRepoNames()` into
`resolveRepoNames()` to avoid misunderstanding since getters are
expected to not mutate their input data in general.
Signed-off-by: Hang Park <hangpark@kaist.ac.kr>