Gradle plugin publishing setup

[yigit]

This CL updates the build to properly publish the gradle
plugin with an id to avoid the need to modify settings
gradle.

After this change, we'll have the following 4 artifacts:

symbol-processing-api: same as before
symbol-processing: does not include the the gradle-plugin anymore
symbol-processing-gradle-plugin: the gradle plugin
com.google.devtools.ksp.gradle.plugin: gradle plugin marker artifact

After this change, developer can enable KSP by:

id("com.google.devtools.ksp")

• Still needs to have google as a plugin repository.

The gradle plugin publishing model requires publishing a marker artifact
so that Gradle can find the main plugin artifact.
I've added java-gradle-plugin plugin to the gradle-plugin module to
enable publishing. Unfortunately, it does have a validation step which
fails while traversing kotlin classes in the classpath so I had to
disable it. We can enable it once gradle depends on kotlin 1.4.

As part of this, I've removed the gradle plugin from the
symbol-processing artifact. Instead, it is now published as
symbol-processing-gradle-plugin artifact (it is consistent with the
project's and androidx's naming scheme but open to changes).

For plugin id, I couldn't use symbol-processing because that would
require us to publish an artifact with symbol-processing group.
Instead, I've picked com.google.devtools.ksp which seems close to what
AGP does (com.android.application). This way, we can publish the marker
artifact in the KSP group.

I've made a couple more changes:

• Updated gradle to 6.6.1, which is the version kotlin uses
• Moved publishing setup to the main gradle file to configure common
  things (pom, version etc)
• Moved the Jar manifest configuration to the main build file so that
  all artifacts have it (was also a necessary change after unbundling the
  gradle plugin since it uses that information to find the version).
  Had to remove Implementation-Title as archiveBaseName is depreacted an
  will be removed in 7.
• Added an outRepo command line argument to set the repository
  location for publication. It is handy when testing with a local build.

Fixes: #203

[yigit]

FYI i didn't update the README since it is a breaking change. We should remember to update it before shipping a version with this :)

[ting-yuan]

Thanks! Will merge before next release.