Use custom pip

[airportyh]

Why?

1. The 1.5 line of poetry does without pip by default (it uses installer package instead). We still need to use pip if we are to leverage the functionality of our customized version of pip. This can be switched off by setting POETRY_INSTALLER_MODERN_INSTALLATION to 0. However,
2. When it does use pip, it does not call it in a way that can use our customized pip. It:
   • uses the python -m pip syntax, which requires the pip library to be in PYTHONPATH
   • uses its own embedded version of pip
   • it passes the --prefix parameter to pip, setting it to the system path, which for us is non-writable and conflicts with the user mode installation

I have considered making these changes based on 1 or 2 configuration flags so they can be upstreamed easier, but I am holding off because that would require a larger change (requiring various functions and their call sites to add a config parameter). I'll try to make this change in their current line (1.6 alpha) in the future.

Changes

1. changed the pip command to simply ["pip"] if POETRY_PIP_FROM_PATH env var is set to "1"
2. removed the --prefix arg when calling pip if POETRY_PIP_NO_PREFIX is set to "1"

[ryantm]

Looks good.

[airportyh]

I've put the new behavior behind env var flags so that:

• ci can pass
• we do not change poetry's behavior unless we opt in to them

[ryantm]

Do you still need this now?

[airportyh]

That's convient for testing. In fact I'll add the 2 new ones...