Missing source files when building package installed from GitHub

I'm running into an issue where the build script is failing for a package that is installed via git/github. The error message looks like this:

```
> angular2-token@0.2.0-beta.13 postinstall /Users/clay/dev/code/tmp/postinstall-test/node_modules/angular2-token
> postinstall-build lib --script=build --verbose

> angular2-token@0.2.0-beta.13 prepublish /Users/clay/dev/code/tmp/postinstall-test/node_modules/angular2-token
> npm run build

> angular2-token@0.2.0-beta.13 build /Users/clay/dev/code/tmp/postinstall-test/node_modules/angular2-token
> rimraf lib && ngc -p src

Error: ENOENT: no such file or directory, lstat 'src'
at Object.fs.lstatSync (fs.js:947:11)
at Object.main (/Users/clay/dev/code/tmp/postinstall-test/node_modules/angular2-token/node_modules/@angular/tsc-wrapped/src/main.js:59:21)
at main (/Users/clay/dev/code/tmp/postinstall-test/node_modules/angular2-token/node_modules/@angular/compiler-cli/src/main.js:19:16)
at Object. (/Users/clay/dev/code/tmp/postinstall-test/node_modules/angular2-token/node_modules/@angular/compiler-cli/src/main.js:35:5)
at Module._compile (module.js:573:30)
at Object.Module._extensions..js (module.js:584:10)
at Module.load (module.js:507:32)
at tryModuleLoad (module.js:470:12)
at Function.Module._load (module.js:462:3)
at Function.Module.runMain (module.js:609:10)
Compilation failed
```

This only fails when I'm trying to install from Git:
```
npm i --save MadeByMunstersLLC/angular2-token#fix_postinstallIssue
```

When the installation is from my local repo, it works perfectly fine:
```
npm i --save /path/to/MadeByMunstersLLC/angular2-token
```

The `postinstall` and `build` scripts for the (forked) angular2-token project are pretty straightforward:

```
"scripts": {
"test": "karma start",
"build": "rimraf lib && ngc -p src",
"prepublish": "npm run build",
"postinstall": "postinstall-build lib --script=build --verbose"
},
```

I've tried with a number of different npm versions, and each results in the same error. I forgot to keep track of exactly which versions, but IIRC they included 5.4, 5.3, 4.6, 4.2, and 3.10. I've reverted to node 8.4.0 and npm 5.3.0, where I'm still seeing the problems.

Seems like this issue might be similar (or identical) to #16, but that issue doesn't mention the package source, and as that seems to be playing a significant role in determining where `postinstall-build` can work with npm to successfully build a package, I figured I'd open up this new issue.

Any thoughts on this build error?