parse index.yaml should use local cache

[bjwswang]

@zqq454224016 pls work with @Abirdcfly on this issue.

[Abirdcfly]

before #84, We are actually parsing the index.yaml file of the repo directly using go's http library, the code can be seen roughly as follows

core/pkg/repository/chartmuseum.go

Line 205 in 90a8097

func (c *chartmuseum) fetchIndexYaml() (*hrepo.IndexFile, error) {

.

After the #84 merge, we introduced the helm.sh/helm/v3 library, which actually behaves the same as the helm binary. When adding a new repository, the helm repo add command actually takes effect, the index.yaml file for the helm repository is already downloaded locally, and the helm.sh/helm/v3 library function provides functions to parse index.yaml, so we can use the helm.sh/helm/v3 library function directly, without having to use go's http library to parse it again.

[zqq454224016]

The file index.yaml exits in pah "helm env : HELM_REPOSITORY_CACHE". "helm env --repository-cache string" can change default path.