pdm import overwrites previous sections of pyproject.toml

[Lyonsclay]

• I have searched the issue tracker and believe that this is not a duplicate.

Make sure you run commands with -v flag before pasting the output.

Steps to reproduce

pdm import app/requirements.txt --format requirements --verbose

./pyproject.toml

[project]
dependencies = [
    "boto3==1.17.15",
    "botocore==1.20.15",
    "elasticsearch==7.11.0",
    "pandas==1.2.2",
]
dev-dependencies = []

[tool]
[tool.pdm]
[[tool.pdm.source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true
[build-system]
requires = ["pdm-pep517"]
build-backend = "pdm.pep517.api"

pdm import app/requirements-dev.txt --format requirements --dev --verbose

[project]
dependencies = []
dev-dependencies = [
    "black",
    "flake8",
    "pytest",
]

[tool]
[tool.pdm]
[[tool.pdm.source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true
[build-system]
requires = ["pdm-pep517"]
build-backend = "pdm.pep517.api"

Actual behavior

The first pdm import command correctly installs the dependencies list.
The second pdm import command correctly installs the dev-dependencies list but also remove the contents of the dependencies list.

Expected behavior

I would expect that subsequent import statements would preserve the content of the other sections.

Environment Information

# Paste the output of `pdm info && pdm info --env` below:

PDM version:        1.4.5                                  
Python Interpreter: /Users/clay/.pyenv/shims/python (3.8.0)
Project Root:       /Users/clay/freakout  
{
  "implementation_name": "cpython",
  "implementation_version": "3.8.0",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "20.3.0",
  "platform_system": "Darwin",
  "platform_version": "Darwin Kernel Version 20.3.0: Thu Jan 21 00:07:06 PST 2021; root:xnu-7195.81.3~1/RELEASE_X86_64",
  "python_full_version": "3.8.0",
  "platform_python_implementation": "CPython",
  "python_version": "3.8",
  "sys_platform": "darwin"
}