Poetry doesn't check extra sources for package

[JaviFuentes94]

• [x] I am on the latest Poetry version.
• [x] I have searched the issues of this repo and believe that this is not a duplicate.
• [x] If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Ubuntu 20.04.3 LTS (Focal Fossa)
• Poetry version: 1.1.12
• Contents of your pyproject.toml file:

  
  [tool.poetry]
  name = "shit"
  version = "0.1.0"
  description = ""
  authors = ["None"]

[tool.poetry.dependencies] python = "^3.7" tensorflow = "1.15.5"

[tool.poetry.dev-dependencies]

[[tool.poetry.source]] name = "tensorflow" url = "https://tf.kmtea.eu/whl/stable.html"

[build-system] requires = ["poetry-core>=1.0.0"] build-backend = "poetry.core.masonry.api"

## Issue
Poetry is incapable of finding a package from a custom source. The package is also part of `pypi`, but not for arm64 architectures, so I need to add a custom source to search for it, but it doesn't try to look for it there. This results on `poetry add tensorflow==1.15.5` failing on arm64 architectures. 

I have added a custom source where this is available (and it works if you do `pip install tensorflow==1.15.5 -f https://tf.kmtea.eu/whl/stable.html`) but poetry never looks in that source for the package, so the build fails with:
```python
Unable to find installation candidates for tensorflow (1.15.5)

  at ~/.poetry/lib/poetry/installation/chooser.py:74 in choose_for
       70│             links.append(link)
       71│
       72│         if not links:
       73│             raise RuntimeError(
    →  74│                 "Unable to find installation candidates for {}".format(package)
       75│             )
       76│
       77│         # Get the best link
       78│         chosen = max(links, key=lambda link: self._sort_key(package, link))

I have also tried with:

[tool.poetry.dependencies]
python = "^3.7"
tensorflow = {version = "1.15.5", source = "tensorflow"}

but it doesn't work either.

Any tips? Thanks!

[JaviFuentes94]

Related #4659

[stumpylog]

I'm assuming this is still an issue? It basically prevents the usage of poetry for projects which support multiple arches but don't want to compile packages which can be found compiled in another repository (example scipy or numpy for armv7).

[IvanaGyro]

This seems still an issue now on the master branch.

[xbug42]

Is there any plan to fix this ? This is Just to know if I should wait for a fix or not... Thx !

[dimbleby]

this is not broken and should be closed

[tool.poetry.dependencies]
python = "^3.10"
tensorflow = { version = "1.15.5", source = "tensorflow" }

[[tool.poetry.source]]
name = "tensorflow"
url = "https://tf.kmtea.eu/whl/stable.html"
priority = "explicit"

is the current syntax