fix: yarn_install failure if yarn is a dependency (#1581)

This fixes a regression in 1.2.0 where the repository rule generated shell script `yarn` in the root of the external repository created by yarn_install conflicts with the npm package `yarn` directory that generate_build_files.js wants to create. The error observed was:
```
Error: ENOTDIR: not a directory, mkdir 'yarn/bin'
    at Object.mkdirSync (fs.js:823:3)
    at mkdirp (/private/var/tmp/_bazel_pryan/7a08068b62d3caa281adb87aa0ce207a/external/npm/generate_build_file.js:81:16)
    at writeFileSync (/private/var/tmp/_bazel_pryan/7a08068b62d3caa281adb87aa0ce207a/external/npm/generate_build_file.js:89:9)
    at generatePackageBuildFiles
```

Also fixes the same potential problem with the repository rule generated `npm` shell script.

internal/npm_install/npm_install.bzl

@@ -219,8 +219,9 @@ def _npm_install_impl(repository_ctx):
 
     # The entry points for npm install for osx/linux and windows
     if not is_windows_host:
+        # Prefix filenames with _ so they don't conflict with the npm package `npm`
         repository_ctx.file(
-            "npm",
+            "_npm.sh",
             content = """#!/usr/bin/env bash
 # Immediately exit if any command fails.
 set -e
@@ -234,7 +235,7 @@ set -e
         )
     else:
         repository_ctx.file(
-            "npm.cmd",
+            "_npm.cmd",
             content = """@echo off
 cd "{root}" && "{npm}" {npm_args}
 """.format(
@@ -264,7 +265,7 @@ cd "{root}" && "{npm}" {npm_args}
 
     repository_ctx.report_progress("Running npm install on %s" % repository_ctx.attr.package_json)
     result = repository_ctx.execute(
-        [repository_ctx.path("npm.cmd" if is_windows_host else "npm")],
+        [repository_ctx.path("_npm.cmd" if is_windows_host else "_npm.sh")],
         timeout = repository_ctx.attr.timeout,
         quiet = repository_ctx.attr.quiet,
     )
@@ -338,8 +339,9 @@ def _yarn_install_impl(repository_ctx):
 
     # The entry points for npm install for osx/linux and windows
     if not is_windows_host:
+        # Prefix filenames with _ so they don't conflict with the npm packages
         repository_ctx.file(
-            "yarn",
+            "_yarn.sh",
             content = """#!/usr/bin/env bash
 # Immediately exit if any command fails.
 set -e
@@ -353,7 +355,7 @@ set -e
         )
     else:
         repository_ctx.file(
-            "yarn.cmd",
+            "_yarn.cmd",
             content = """@echo off
 cd "{root}" && "{yarn}" {yarn_args}
 """.format(
@@ -383,7 +385,7 @@ cd "{root}" && "{yarn}" {yarn_args}
 
     repository_ctx.report_progress("Running yarn install on %s" % repository_ctx.attr.package_json)
     result = repository_ctx.execute(
-        [repository_ctx.path("yarn.cmd" if is_windows_host else "yarn")],
+        [repository_ctx.path("_yarn.cmd" if is_windows_host else "_yarn.sh")],
         timeout = repository_ctx.attr.timeout,
         quiet = repository_ctx.attr.quiet,
     )

package.json

@@ -49,6 +49,7 @@
         "node_resolve_main": "file:./tools/npm_packages/node_resolve_main",
         "node_resolve_main_2": "file:./tools/npm_packages/node_resolve_main_2",
         "node_resolve_nested_main": "file:./tools/npm_packages/node_resolve_nested_main",
+        "npm": "6.13.7",
         "parse5": "5.1.0",
         "protobufjs": "6.8.8",
         "protractor": "^5.4.2",
@@ -74,6 +75,7 @@
         "typescript": "3.1.6",
         "unidiff": "1.0.1",
         "v8-coverage": "1.0.9",
+        "yarn": "1.21.1",
         "zone.js": "0.8.29"
     },
     "scripts": {