chore/Scrutinize pyproject for backends

[blast-hardcheese]

Why

In cases where users are using pyproject.toml for more than just Poetry, we should permit pip + pyproject to coexist.

What changed

• Introduced an optional IsSpecfileCompatible to analyze the contents of a specfile to confirm compatibility. Currently exclusively for poetry, but this could extend to package.json as well

Test plan

HD73VL2GVX:pyprpip dstewart$ cat pyproject.toml
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

HD73VL2GVX:pyprpip dstewart$ upm lock
could not autodetect a language for your project

HD73VL2GVX:pyprpip dstewart$ touch requirements.txt

HD73VL2GVX:pyprpip dstewart$ upm lock
--> pip install -r requirements.txt
...