Enhanced check_requirements() with auto-install (ultralytics#2575)

* Update check_requirements() with auto-install

This PR builds on an idea I had to automatically install missing dependencies rather than simply report an error message. 

YOLOv5 should now 1) display all dependency issues and not simply display the first missing dependency, and 2) attempt to install/update each missing/VersionConflict package.

* cleanup

* cleanup 2

* Check requirements.txt file exists

* cleanup 3

(cherry picked from commit 2b329b0)

utils/general.py

@@ -1,4 +1,4 @@
-# General utils
+# YOLOv5 general utils
 
 import glob
 import logging
@@ -86,10 +86,20 @@ def check_git_status():
 
 def check_requirements(file='requirements.txt', exclude=()):
     # Check installed dependencies meet requirements
-    import pkg_resources
-    requirements = [f'{x.name}{x.specifier}' for x in pkg_resources.parse_requirements(Path(file).open())
-                    if x.name not in exclude]
-    pkg_resources.require(requirements)  # DistributionNotFound or VersionConflict exception if requirements not met
+    import pkg_resources as pkg
+    prefix = colorstr('red', 'bold', 'requirements:')
+    file = Path(file)
+    if not file.exists():
+        print(f"{prefix} {file.resolve()} not found, check failed.")
+        return
+
+    requirements = [f'{x.name}{x.specifier}' for x in pkg.parse_requirements(file.open()) if x.name not in exclude]
+    for r in requirements:
+        try:
+            pkg.require(r)
+        except Exception as e:  # DistributionNotFound or VersionConflict if requirements not met
+            print(f"{prefix} {e.req} not found and is required by YOLOv5, attempting auto-install...")
+            print(subprocess.check_output(f"pip install '{e.req}'", shell=True).decode())
 
 
 def check_img_size(img_size, s=32):