This should allow you to continue mapping for Counter-Strike: Source while valve attempts to fix a massive flaw in the steampipe content update.
DO NOT SKIP ANY STEPS OR TEXT OR ELSE YOU MAY NOT DO IT CORRECTLY
Part 1 - Redirecting the old counter strike source path to the new one
1. Go to steam/steamapps/username and create a folder called "counter-strike source"
2. Open the command prompt and then put these commands(insert username where I put <username> and do each line indvidually - do not enter all 3 lines at once) in order(if you use a different drive/directory then change it accordingly):
cd c:\program files\steam\steamapps\<username>"
rmdir "counter-strike source"
mklink /j "cstrike" c:\program files\steam\steamapps\common\counter-strike source\"
Note: This will redirect everything that reads the old counter-strike source folder to the new one. This step may not be necessary to do, but I haven't had time to check and confirm it.
Part 2 - Setting hammer to read the old gameinfo file
1. Download this:
Download gameinfo.txt from Sendspace.com - send big files the easy way
2. Put it in any folder that you wish(for this tutorial I'm just going to be putting it in steamapps/username/counter-strike source2/cstrike2 since that's what I did) but make sure that it's the ONLY FILE in the folder.
3. Create 3 folders and name them: materials | models | sound
4. Open the command prompt and put these commands(again, do each line individually):
cd c:\program files\steam\steamapps\<username>\counter-strike source2\cstrike2"
rmdir "materials"
mklink /j "cstrike" c:\program files\steam\steamapps\common\counter-strike source\cstrike\materials\"
cd c:\program files\steam\steamapps\<username>\counter-strike source2\cstrike2"
rmdir "models"
mklink /j "cstrike" c:\program files\steam\steamapps\common\counter-strike source\cstrike\models\"
cd c:\program files\steam\steamapps\<username>\counter-strike source2\cstrike2"
rmdir "sound"
mklink /j "cstrike" c:\program files\steam\steamapps\common\counter-strike source\cstrike\sound\"
5. Open hammer, go to the 'tools' menu and click options
6. In the 'game configurations' tab, go to the 'game directory' box and then set the path to where you placed the old gameinfo.txt file(e.g for me I'll put $SteamUserDir/counter-strike source2/cstrike2 because that's where I placed it and $SteamUserDir just means the <username> folder)
Note: If this doesn't work, you probably made a small error - this needs to be done very precisely. It worked for me, so I'm unsure as to why it wouldn't for others.
I'll update this post if valve makes any changes.