Combine repository override configuration with file existence

[anatoliykmetyuk]

First step to solving sbt/sbt#9369. Second part of the fix is sbt/sbt#9452.

Prior work that changed the JVM flag processing semantics, thereby causing the issue: sbt/sbt#8761

The processing pipeline involved in the issue:

1. sbt-launcher - configures the launcher to launch sbt. Propagates user-supplied configuration to the launcher:
   • config fieldoverride-build-repos: JVM flag sbt.override.build.repos=true
   • config field repository-override: File repositories_force
2. launcher - consumes sbt-launcher-supplied config. Outputs isOverrideRepositories boolean flag based on the data supplied by (1) sbt-launcher.
3. Project-specific sbt runtime. Launcher by the launcher. Sets a setting whether or not to override repos.
   • sbt 1: sets it to (2) launcher output flag.
   • sbt 2: (2) launcher output flag OR (1) JVM flag sbt.override.build.repos=true

The fix is two-fold:

1. Teach launcher that repos are meant to be overridden either on flag or file presence. isOverrideRepositories = configuredOverrideRepos || repoConfigOverrideExists.
2. In the sbt-launcher template, restore override-build-repos that was removed in https://github.com/sbt/sbt/pull/8761/changes#diff-f12f6468a3b483d79284e248213d4c1897f90ab80159db60343f9d907abf5188L28. This would propagate the JVM flag to the launcher to be used in isOverrideRepositories computation.

This PR is the implementation of the part 1 of the fix. Once launcher is released, we can bump the launcher version on sbt/sbt and implement step (2).

[eed3si9n]

Thanks!