[Installer] Load '*.podspec.json' file from `:path` directive.

I've created a Podfile the following:

```
# Podfile
pod 'MyPrivateModule', :path => '~/src/MyPrivateModule' 
```

`MyPrivateModule.podspec.json` is exist in this directory, 
but `MyPrivateModule.podspec(RUBY)` is not exist.

And, `pod install` failed in this situation.

```
# Console Out
Preparing
Analyzing dependencies
Fetching podspec for `MyPrivateModule` from `/Users/laiso/src/MyPrivateModule`
[!] No podspec found for `MyPrivateModule` in `/Users/laiso/src/MyPrivateModule`
```

I think this problem to be resolved by change to `external_sources/path_source.rb`.

* JSON spec can not be found using :path option · Issue #2331