How do I find the package that provides a file?

You can use dpkg command to find out which installed package owns a file:

From man dpkg:

-S, --search filename-search-pattern...
                  Search for a filename from installed packages.

Example:

$ dpkg -S /bin/ls
coreutils: /bin/ls

You can either search with a full path or with just the filename.

If you wish to search for files not yet installed on your computer, you can use the Ubuntu Packages Search
or apt-file as described in a different answer.

The apt-file command can do this for you from the command line. I use it frequently when building packages from source. For files provided by packages that are already installed on your system, apt-cache is another choice.

To install apt-file, do:

sudo apt-get install apt-file

Then, you need to update it's database:

sudo apt-file update

And, finally, search the file:

$ apt-file find kwallet.h
kdelibs5-dev: /usr/include/kwallet.h
libkf5wallet-dev: /usr/include/KF5/KWallet/kwallet.h

However a much friendlier way is to use the Ubuntu Packages Search website. They have an option to "search the contents of packages" for a specific filename.

There's also apt-file for looking up files in packages that aren't installed. For example:

apt-file list packagename