Move core Bazelisk logic into core package. (bazelbuild#166)

The bazelisk.go file now only contains the main function, since all of its previous code was moved into core/core.go.

As a result, Bazelisk can now be used as a library with custom repository implementations, with core.RunBazelisk() serving as entry point.

BUILD

@@ -45,13 +45,8 @@ go_library(
     visibility = ["//visibility:private"],
     x_defs = {"BazeliskVersion": "{STABLE_VERSION}"},
     deps = [
-        "@com_github_hashicorp_go_version//:go_default_library",
-        "@com_github_mitchellh_go_homedir//:go_default_library",
         "//core",
-        "//httputil",
-        "//platforms",
         "//repositories",
-        "//versions",
     ],
 )
 
@@ -64,6 +59,7 @@ go_test(
     embed = [":go_default_library"],
     importpath = "github.com/bazelbuild/bazelisk",
     deps = [
+        "//core",
         "@io_bazel_rules_go//go/tools/bazel:go_default_library",
     ],
 )