LinkageCheckRequest and LinkageCheckRequest.Builder

Current design of creating LinkageChecker for every class path looks strange. Let's change the parameters into `LinkageCheckRequest` class.

```
LinkageChecker linkageChecker = LinkageChecker.create();
LinkageCheckRequest.Builder request = LinkageCheckRequest.builder()
.classPath(classPath)
.exclusionFile(exclusionFile);

// current findSymbolProblems()
ImmutableSetMultimap = linkageChecker.check(request.build());
```

The `linkageChecker` instance does not hold any information. Maybe a static method should suffice:

```
LinkageCheckRequest.Builder request = LinkageCheckRequest.builder()
.classPath(classPath)
.exclusionFile(exclusionFile);
ImmutableSetMultimap result = LinkageChecker.check(request.build());
```