Initial rfc for pip install buildpack (#2)

Author: emmjohnson

rfcs/0001-pip-install-rearchitecture.md

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+# Pip Install Architecture
+
+## Proposal
+The Pip Install buildpack will use the package installer
+[pip](https://pypi.org/project/pip) to install dependencies from a
+`requirements.txt` file into a layer that will be managed by the buildpack.
+
+## Motivation
+In keeping with the overarching [Python Buildpack Rearchitecture
+RFC](https://github.com/paketo-community/pip/blob/main/rfcs/0001-pip-rearchitecture.md),
+the Pip Install Buildpack should perform one task, which is installing from
+requirements files. This is part of the effort in Paketo Buildpacks to reduce
+the responsibilities of each buildpack to make them easier to understand and
+maintain.
+
+## Implementation
+### API
+- pip-install
+  - `requires`: `cpython` and `pip` during build
+  - `provides`: `site-packages`
+
+### Detect
+The pip-install buildpack should only detect if there is a `requirements.txt`
+file at the root of the app.
+
+### Build
+There will be two layers, `packages` layer and `cache` layer.
+The packages layer will contain the result of the pip install command.
+The cache layer will hold the pip [cache](https://pip.pypa.io/en/stable/reference/pip_install/#caching).
+
+During the build process, it will utilize the `pip` tool provided by the `pip`
+requirement (e.g. provided by the `paketo-community/pip` buildpack).
+
+The resulting build command run by the Pip Install buildpack will be:
+```bash
+pip install
+  --requirement <requirements file>           # install from given requirements file
+  --ignore-installed                          # ignores previously installed packages
+  --exists-action=w                           # if path already exists, wipe before installation
+  --cache-dir=<path to cache layer directory> # reuse pip cache
+  --compile                                   # compile python source files to bytecode
+  --user                                      # install to python user install directory set by PYTHONUSERBASE
+  --disable-pip-version-check                 # ignore version check warning
+```
+Upgrade options are ignored if using `--ignore-installed` See [upgrade
+options](https://pip.pypa.io/en/stable/development/architecture/upgrade-options/).
+
+This should be run with the environment variable `PYTHONUSERBASE` set to the packages layer directory.
+
+If the app has a vendor directory at the root, the app will be considered vendored and the resulting build command will be:
+```bash
+pip install
+  --requirement <requirements file>
+  --ignore-installed
+  --exists-action=w
+  --no-index                                   # ignore package index, uses --find-links URLs
+  --find-links=<file://<app vendor directory>> # uses apps vendor directory
+  --compile
+  --user
+  --disable-pip-version-check
+```
+
+#### Environment variables
+
+The buildpack should attach the environment variable `$PYTHONUSERBASE` to the
+`packages` layer, set its value to the layer's directory path, and make it
+available during both the build and launch phases.