Part of [CMP-10017](https://youtrack.jetbrains.com/issue/CMP-10017)
Simplify merges short-term. Make buildSrc/build.gradle for the fork mode

The reason is to simplify regular AOSP merges.

There are 2 modes:
- AOSP mode. used if it is opened via `gradlew studio`
- fork mode. used if it is opened via IDEA.

The build fork is separated using this code:
```
def isRunFromGradlewStudio = System.getenv().get("EXPECTED_AGP_VERSION")
if (!isRunFromGradlewStudio) {
    apply from: "settings-fork.gradle"
    return
}
```

This works, but not ideally because some parts of the build are still
shared with AOSP and we need to review them during the merge:
```
gradle.properties
gradle/wrapper
gradle/libs.version.toml
buildSrc/gradle.properties
```
Alternatives that I tried:
- create `fork-project`, add symlinks to the top-level projects. Didn't
work because integration with Git is broken (the changed files are
treated as not a part of the project)
- create `fork-project`, use `../project` paths to the top-level
projects. Didn't work because running from the gutter is broken (IDEA
tells that the project folder isn't part of the root project)

Next steps:
1. [Support using sources instead of androidx
artifacts](#3169)
2. Merge it into `integration`
3. Reset state of `buildSrc`, `build.gradle` to the AOSP state
(excluding the code that redirects to the fork)
4. Add an automatic task that compares source sets dependencies inside
`build-fork.gradle`
5. Clean up `buildSrc` from not needed code


[TeamCity](https://teamcity.jetbrains.com/buildConfiguration/JetBrainsPublicProjects_Compose_AllPersonalBuild/6263205)

Should be reviewed commit by commit.

## Release Notes
N/A