This was needed to get correct sha for requirements.yaml and requirements.lock
Fixes https://github.com/kubernetes/helm/issues/2598

```
- name: <dependency-chart-name>
  alias: <alias-name-to-be-used>
  version: <dependency-chart-version>
  repository: <dependency-chart-version>
```

Fixes https://github.com/kubernetes/helm/issues/2383

Sample `requirements.yaml` I used for test purpose was for wordpress chart

```dependencies:
- name: mariadb
  version: 0.5.10
  repository: https://kubernetes-charts.storage.googleapis.com/
  alias:
  - db1
  - db2
  - db3
```

support repo alias in requirements.yaml
The syntax should be like: "alias:reponame" or "@reponame".

closes: #1985

Implements a mechanism in requirements.yaml to allow the import
and re-parenting of value table from child chart.

Closes #1995

closes bug: 2103

When downloader package moved to under /pkg, helmpath functions got called still from it, while it is under /cmd. This commit fixes the issue by moving helmpath after the downloader to have only cmd->pkg and pkg->pkg calls.

nit: fix spelling of dependency

fix change requests

The following commands:
helm dep update
helm dep build
are now able to take a requirements.yaml with dependency charts' repo defined as:
file://../local/path or file:///root/path

closes: #1884

This feature adds the ability to selectively control the loading of charts using entries in top chart's values.
When 'helm install --set tags.mytag=true', charts with that tag will be enabled unless disabled in parent by condition.
When 'helm install --set mychart.enabled=true', charts with that yaml path specified will be enabled.

Closes #1837