Allow using Resolver/Publisher independently

[iRevive]

Currently, it's hard to use a plugin only for publishing. Especially when the publishing process happens only on CI. At this point, a valid token is not needed locally.

But the build will fail anyway since the credentials setting requires a token to be defined.

From my point of view, the functionality can be separated into smaller modules.

Project modules:

• sbt-github-packages-common - shared modules: TokenSource, GitHubRepository, utility methods
• sbt-github-packages-publisher - only Publisher
• sbt-github-packages-resolver - only Resolver
• sbt-github-packages - both Publisher and Resolver. Identical to the current definition.

Publisher

Responsible for the publishing part.

Settings:

• githubSuppressPublicationWarning
• githubPublishToRepository - The entry is a combination of owner, repository, and token source.

  githubPublishToRepository := GitHubRepository(
  "djspiewak", 
  "sbt-github-packages", 
  TokenSource.Environment("GH_PACKAGES_TOKEN")
  )

Derived settings (based on githubPublishToRepository):

• githubPublishTo
• scmInfo
• homepage
• pomIncludeRepository
• publishMavenStyle

Behavior:

• update - does not fail if a token is missing
• publish - may fail if a token is missing

Resolver/Downloader/Loader

The should be a better name I believe.

It would be nice if multiple GitHub repositories can be supported.

Settings:

• githubRepositories - The GitHub repositories. The entry is a combination of owner, repository, and token source.

  githubRepositories += GitHubRepository(
  "djspiewak", 
  "sbt-github-packages", 
  TokenSource.Environment("GH_TOKEN")
  )

Derived settings (based on githubRepositories):

• resolvers
• credentials

Behavior:

• update - may fail if a token is missing

[colindean]

I just had to work around this problem with https://github.com/target/data-validator/pull/98 by setting an invalid token just to allow sbt to run.