Generate BUILD file with aliases for wheels packages

[oxidase]

Add convenience aliases to a BUILD file that is a sibling file to args.output one. Aliases allow to avoid loading @external_pip//:requirements.bzl file and use @external_pip//:pip targets directly. The alias target names are taken from the args.input as-is without lower-casing and have public visibility.

---

This change is 

[ali5h]

would u provide a sample for the new build files?

[oxidase]

for pre-compiled requirements

google_cloud_language
numpy
PyQt5
scipy

the generated BUILD file will be

# Generated BUILD file
[alias(name=name, actual=pkg,  visibility=["//visibility:public"]) for name, pkg in {
  "cachetools": "@pypi__36__cachetools_4_1_1//:pkg",
  "certifi": "@pypi__36__certifi_2020_6_20//:pkg",
  "chardet": "@pypi__36__chardet_3_0_4//:pkg",
  "google-api-core": "@pypi__36__google_api_core_1_21_0//:pkg",
  "google-api-core[grpc]": "@pypi__36__google_api_core_1_21_0//:grpc",
  "google-auth": "@pypi__36__google_auth_1_18_0//:pkg",
  "google-cloud-language": "@pypi__36__google_cloud_language_1_3_0//:pkg",
  "googleapis-common-protos": "@pypi__36__googleapis_common_protos_1_52_0//:pkg",
  "grpcio": "@pypi__36__grpcio_1_30_0//:pkg",
  "idna": "@pypi__36__idna_2_10//:pkg",
  "numpy": "@pypi__36__numpy_1_19_0//:pkg",
  "protobuf": "@pypi__36__protobuf_3_12_2//:pkg",
  "pyasn1": "@pypi__36__pyasn1_0_4_8//:pkg",
  "pyasn1-modules": "@pypi__36__pyasn1_modules_0_2_8//:pkg",
  "pyqt5": "@pypi__36__pyqt5_5_15_0//:pkg",
  "pyqt5-sip": "@pypi__36__pyqt5_sip_12_8_0//:pkg",
  "pytz": "@pypi__36__pytz_2020_1//:pkg",
  "requests": "@pypi__36__requests_2_24_0//:pkg",
  "rsa": "@pypi__36__rsa_4_6//:pkg",
  "scipy": "@pypi__36__scipy_1_5_0//:pkg",
  "setuptools": "@pypi__36__setuptools_47_3_1//:pkg",
  "six": "@pypi__36__six_1_15_0//:pkg",
  "urllib3": "@pypi__36__urllib3_1_25_9//:pkg"
}.items()]

Aliases are somewhat limited wrt the requirement macro because there is no way for the target parameter use case.

[ali5h]

this won't work when there are multiple python versions in the same repo, which is one of the main use cases

[ali5h]

how about adding the aliases under the pip repo (name of the pip_import rule)

[oxidase]

I was not thinking about multiple python runtimes, but this is how it works for google_cloud_language requirement

pip_import(name = "pip35", requirements = "//:requirements.txt", python_interpreter = "python3.5")
load("@pip35//:requirements.bzl", pip35_install = "pip_install")
pip35_install()
pip_import(name = "pip36", requirements = "//:requirements.txt", python_interpreter = "python3.6")
load("@pip36//:requirements.bzl", pip36_install = "pip_install")
pip36_install()

BUILD files will be external/pip35/BUILD

# Generated BUILD file
[alias(name=name, actual=pkg,  visibility=["//visibility:public"]) for name, pkg in {
  "certifi": "@pypi__35__certifi_2020_6_20//:pkg",
  "googleapis-common-protos": "@pypi__35__googleapis_common_protos_1_52_0//:pkg",
  "protobuf": "@pypi__35__protobuf_3_12_2//:pkg",
  "pyasn1": "@pypi__35__pyasn1_0_4_8//:pkg",
  "setuptools": "@pypi__35__setuptools_47_3_1//:pkg",
  "chardet": "@pypi__35__chardet_3_0_4//:pkg",
  "google-cloud-language": "@pypi__35__google_cloud_language_1_3_0//:pkg",
  "grpcio": "@pypi__35__grpcio_1_30_0//:pkg",
  "pyasn1-modules": "@pypi__35__pyasn1_modules_0_2_8//:pkg",
  "urllib3": "@pypi__35__urllib3_1_25_9//:pkg",
  "rsa": "@pypi__35__rsa_4_6//:pkg",
  "cachetools": "@pypi__35__cachetools_4_1_1//:pkg",
  "google-api-core": "@pypi__35__google_api_core_1_21_0//:pkg",
  "six": "@pypi__35__six_1_15_0//:pkg",
  "requests": "@pypi__35__requests_2_24_0//:pkg",
  "pytz": "@pypi__35__pytz_2020_1//:pkg",
  "google-auth": "@pypi__35__google_auth_1_18_0//:pkg",
  "google-api-core[grpc]": "@pypi__35__google_api_core_1_21_0//:grpc",
  "idna": "@pypi__35__idna_2_10//:pkg"
}.items()]

and in external/pip36/BUILD

# Generated BUILD file
[alias(name=name, actual=pkg,  visibility=["//visibility:public"]) for name, pkg in {
  "cachetools": "@pypi__36__cachetools_4_1_1//:pkg",
  "certifi": "@pypi__36__certifi_2020_6_20//:pkg",
  "chardet": "@pypi__36__chardet_3_0_4//:pkg",
  "google-api-core": "@pypi__36__google_api_core_1_21_0//:pkg",
  "google-api-core[grpc]": "@pypi__36__google_api_core_1_21_0//:grpc",
  "google-auth": "@pypi__36__google_auth_1_18_0//:pkg",
  "google-cloud-language": "@pypi__36__google_cloud_language_1_3_0//:pkg",
  "googleapis-common-protos": "@pypi__36__googleapis_common_protos_1_52_0//:pkg",
  "grpcio": "@pypi__36__grpcio_1_30_0//:pkg",
  "idna": "@pypi__36__idna_2_10//:pkg",
  "protobuf": "@pypi__36__protobuf_3_12_2//:pkg",
  "pyasn1": "@pypi__36__pyasn1_0_4_8//:pkg",
  "pyasn1-modules": "@pypi__36__pyasn1_modules_0_2_8//:pkg",
  "pytz": "@pypi__36__pytz_2020_1//:pkg",
  "requests": "@pypi__36__requests_2_24_0//:pkg",
  "rsa": "@pypi__36__rsa_4_6//:pkg",
  "setuptools": "@pypi__36__setuptools_47_3_1//:pkg",
  "six": "@pypi__36__six_1_15_0//:pkg",
  "urllib3": "@pypi__36__urllib3_1_25_9//:pkg"
}.items()]

and aliases are "@pip35//:google-cloud-language" or "@pip36//:google-cloud-language"

[ali5h]

oh, yeah, this build is in that external repo, my bad

[ali5h]

please add this to readme too, very useful. thank you

[oxidase]

added two usage examples

[ali5h]

one more thing, would you change one of the packages in the examples to be used like this? this will act as a test too.

[oxidase]

added "@piptool_deps_tests_3//:numpy"