Adds a wrapper to all datasources to provide a generic purl-based interface.

Closes #1764 

This PR follows up and fixes #1968

The previous PR didn't behave correctly as it was expecting Github Enterprise to ALWAYS been configured and that the github.com token was available in the `GITHUB_COM_TOKEN` env variable.
But for non GHE project `GITHUB_COM_TOKEN` is not defined and github.com token is available at the `GITHUB_TOKEN` env variable instead.

This updated PR fix this issue and avoid further problems by prioritising `github.com` over GHE.
Now the code is NOOP if no `GITHUB_ENDPOINT` is configured.

If it's configured, instead, now the codes assumes that, by DEFAULT, a dependency is hosted on `github.com` so it removes `GITHUB_ENDPOINT` and use `GITHUB_COM_TOKEN` as Github token immediately.
They are restored only if needed, when a dependency is hosted on the provided GithubEnterprise.

This reverts commit 7fcbb47b.

This reverts commit 080d9451.

Closes #1988

This reverts commit f2fb2e6b.

as nexus hosted npm repository does not automatically 'extracts' any value from the published versions we have to manually pick the repository and namepage fields from the latest version if not already defined in the reponse root

npmjs’s CDN is (erroneously) returning 408 Timeout responses sometimes, so these should be retried.

#1407 introduced a bug where the replacement logic breaks if the value of the npmrc key is a boolean.

Adds a config option to bot administrators called `exposeEnv`, for cases where repositories are trusted. If set to true, the bot's full `process.env` can be used for `.npmrc` variable substitution and is passed to child processes when generating lock files. Disabled by default, including in the App.

Renovate now aborts processing of repositories if for any 4xx responses except 401 and 404, and also for 200 OK responses which are unparseable.

Closes #1341